A Changeling is a creature of fantasy, generally an offspring of an otherworldly species left to be raised by parents of a more mundane sort. In modern fiction it is often simply some sort of race of shape shifters, or the name/alias of a single shape shifter.
Being folklore there are many variations, but they all follow the same pattern: a fairy kidnaps a child and replaces it with something known as a changeling. What the changeling is, how it behaves, how long the parent is stuck with it, and how you get rid of it sooner than that (if you should get rid of it), depends on the version. In some versions it's the fairy's offspring; in others, it's a piece of wood with glamour on it. Sometimes to get rid of it you throw it in the oven, other times you be abusive to it and the faerie switches the children again because you aren't taking good care of its child. (In practice, this had a nasty tendency of escalating to full-blown infanticide.) In Ireland it was common to dress up boys as girls up to the age of 12 or 13 because their fairies had a preference of kidnapping boys, and items like iron scissors would often be left where the child sleeps to ward off the fairy. The child that is kidnapped is sometimes also called a changeling, but the term usually refers to whatever the fairy left in the child's place.
Dungeons & Dragons
In Dungeons & Dragons, "Changeling" is a name applied to one of four races; the Cerilian Changeling, the Shadow Fey Changeling, the Eberron Changeling, and the Faerie Changeling.
The World of Cerilia, home of the Birthright setting, is more deeply based on European folklore than standard D&D settings. As such, it is home to a precusor to the Feywild, which is populated by its own unique fey races; the Seelie and the Unseelie. Being based on the old faerie tale lores, these fey sometimes steal away children from human (or, more rarely, elven) families, with a specific breed of child-sized faerie trying to adopt the stolen child's place. The faerie eventually bores of the masquerade (if not discovered and forced to flee sooner) and so leaves of its own accord after six or seven years, whilst the fey-raised child is usually cast out upon reaching puberty and no longer looking as cute. "Changeling" is used to refer both the faerie and the victim in this affair.
Shadow Fey Changeling
In the Demiplane of Dread live a race of Gothic Horror themed fey; the Arak, or Shadow Fey. As such, they have their own form of changelings. These are humans who, for whatever reason, impressed a shadow fey with their skill enough that the shadow fey chose to "preserve" them. Through a complicated ritual that involves feeding the soon-to-be-changeling some fey food to put them into a coma, before lighting a magical candle, sprinkling the victim with special mystic dust, ritually cutting away their shadow with an enchanted silver sickle, bunging the shadow into an enchanted sack, and legging it for the Shadow Rift before the candle burns out (or is extinguished). If the shadow fey succeeds at it, the victim's shadow is transformed into a changeling; an unaging, mindless drone that resembles its "donor", but with a fey trait based on its creator and a permanent vaucous expression, which lives onto to exercise its specific skill, be it medicine, fighting, gardening, baking, smithing, whatever.
As you might expect, the shadow-less donor is not left in a particularly good condition; having lost a fundamental chunk of their soul, these "elf-shot" individuals become mindless zombies, shuffling through life without direction and swiftly starving to death if not cared for. The shadow fey see nothing wrong in what they do; as far as they are concerned, creating a changeling from a mortal's soul is a gift, as it preserves the most useful and valuable part of the mortal for all eternity.
Shadow Fey Changelings are also known as "Kin", referring to which of the breeds created them and which thusly defines their traits; all of the "common" breeds of Arak create them, but it's unknown if "lesser" breeds like the Waff and Gwytune either do or can.
These changelings were around in both Advanced Dungeons & Dragons 2nd edition and in Dungeons & Dragons 3rd Edition, being covered in the Shadow Rift and in Van Richten's Guide to the Shadow Fey sourcebooks, respectively.
The known shadow changeling breeds are:
- Alvenkin: Inoffensive by nature, these changelings serve as farmers, gardeners, herbalists and other horticulturalists, much like the alven do.
- Bragkin: Those who are chosen to become Bragkin changelings are always involved in tasks of physical engineering; they are architects, builders, carpenters, masons... if it involves making things through a lot of heavy labor, then that's the role these changelings fill. As such, they tend to be quite strong.
- Firkin: Artificers and technical artisans are made into changelings to serve the Fir. Clockwork, trap-crafting, anything involving mechanical devices is the domain of these changelings.
- Muryankin: One of the few changeling breeds inclined to violence, Muryankin are the muscle of the Arak, serving as expendable guardians and soldiers through their combination of fighting skill and berserk fury.
- Portunekin: A rare breed indeed, Portunekin were doctors, physicians, and other healers in their old life, which is why they were taken; to become the healers of the Arak.
- Powriekin: The other changeling breed most likely to be sent on missions of blood, Powriekin were changed for their skills in stealth, theft and murder; they work for the Arak in the wider world as thieves and assassins.
- Sheekin: Performers and entertainers of all stripes are snatched up by the Shee, making these soul-less "living performance machines" perhaps the most common changeling breed.
- Sithkin: Pale and ghostly, to the point they are often mistaken by the unwary for the Undead, Sithkin don't have as clear a purpose as the other breeds; they seem to serve the Sith as general dogsbodies, being sent to receive items of importance to their necromancer masters from places of death, such as graveyards and mortuaries.
- Tegkin: As feral as the Tegs that made them, these changelings are the hunters of the Arak, thinking only of bringing down whatever quarry they are directed to, and supplying meat and hides to the Shadow Fey as a whole.
The changeling race that people actually remember was a race introduced in the 3.5 edition campaign setting of Eberron; this version of the changeling was portrayed as being a race of humanoids with a hint of doppelganger in their ancestry, which gives them limited shapeshifting abilities. In their debuting edition this takes the form of their signature supernatural racial feature, Minor Change Shape. In major cities they are often prostitutes, able to be anything you want (at least so long as it's between 4'3 and 6'9 and humanoid). They have a reasonably sized population and make 2-4% of the population of major cities. They were reprinted in Monster Manual III with a nod that they exist in other settings, though unlike Warforged though they don't get any new options there.
Since they're a core race in Eberron's campaign setting they got a lot of support within Eberron supplements and even books outside them actually remembered they existed. They have two unique Prestige Classes (and a bunch of feats) in Races of Eberron: Cabinet Trickster and Recaster. Cabinet Trickster makes them equal to a true doppleganger in shapeshifting and mindreading ability but otherwise not impressive. Recaster however is one of the few casting prestige classes that loses caster levels and is still worth it thanks to some spontaneous and free metamagic a limited number of times per day and the ability to pick two spells from any casting list in the game (Haste as a level 1 spell from Trapsmith). They also got three racial substitution levels consisting of some minor stuff for Egoist and more interesting stuff for Rogue and Wizard. Changling Rogue replaces trapfinding with 10 skill points a level and some extra social abilities, making it an awesome first level dip. More unusual is the ability for Wizards that specialize in transmutation and illusion (You still get the same number of slots but lose an extra school, and transmutation has plenty of options) while attracting a familiar as morphic as they are. Outside of the two classes from Races of Eberron, there's an extra pair of prestige classes that aren't exclusive to it but it is uniquely suited for: Warshaper and Chameleon (detailed on the prestige class page).
Brought to 5e with the Eberron Unearthed Arcana. +1 Dexterity and Charisma, Duplicity (Deception proficiency), Shapechanger (polymorph into medium-sized human that you've seen, but equipment doesn't adjust with you), two bonus languages, 30 ft speed.
|Dungeons & Dragons 4th Edition Races|
|Player's Handbook 1:|| Dragonborn - Dwarf - Eladrin - Elf |
Half-Elf - Halfling - Human - Tiefling
|Player's Handbook 2:||Deva - Gnome - Goliath - Half-Orc - Shifter|
|Player's Handbook 3:||Githzerai - Minotaur - Shardmind - Wilden|
|Monster Manual 1:|| Bugbear - Doppelganger - Githyanki |
Goblin - Hobgoblin - Kobold - Orc
|Monster Manual 2:||Bullywug - Duergar - Kenku|
|Dragon Magazine:||Gnoll - Shadar-kai|
|Heroes of Shadow:||Revenant - Shade - Vryloka|
|Heroes of the Feywild||Hamadryad - Pixie - Satyr|
|Eberron's Player's Guide:||Changeling - Kalashtar - Warforged|
|The Manual of the Planes:||Bladeling|
|Dark Sun Campaign Setting:||Mul - Thri-kreen|
|Forgotten Realms Player's Guide:||Drow - Genasi|
This obscure race was introduced in Dragon Magazine #304, and was essentially the classic fairytale version made playable: a fey humanoid that was given to mortal parents to raise, making it weaker than the rest of its kin. This changeling looked like an attractive human, half-elf or even a full-blooded elf, with at least one subtly odd feature hinting at its true fey origins. This "faerie changeling" was likely a reference to the Cerilian Changeling, and had the following statblock:
- -2 Strength, +2 Dexterity, -2 Constitution, +2 Charisma
- Base speed 30 feet
- Low-light vision
- A changeling is considered both a fey and a humanoid. It can wield fey-restricted weapons and magical items, but is also affected by both fey-bane weapons and by spells that target humanoids, such as Charm Person.
- +2 racial bonus against saving throws that specifically target humanoids, such as Charm Person.
- +2 racial bonus to Disguise checks.
- Fey Luck: A changeling with an Intelligence score of 10 or more has the spell-like abilities of Guidance 1/day, Resistance 1/day and Virtue 1/day. All SLAs are cast as if by a 1st level Sorcerer.
- Favored Class: Bard
In Pathfinder, Changelings are an all-female race first introduced in the Carrion Crown adventure path. These are the juvenile forms of Hags -- read, creepy-ass evil spell-casting monster-crone old women whose hobbies include eating people, murder and torture -- and consequently inherit a lot of spooky behavior, magical powers and forms that are "beautiful, yet unsettling". Hags deliberately foster out their daughters to places where they're confident the locals will mistreat and abuse them; a changeling can only become a hag if she chooses to follow her mother's magical call and agrees to undergo the agonizingly painful ritual to be transformed into a hag. Thusly, mother hags do their best to ensure their daughters get bullied so badly they will willingly grab at the power and the chance to avenge themselves on humanity, even at the cost of being painfully turned into a hideously ugly old witch. Changelings have a natural affinity for the Witch class, as one might expect.
Changelings get +2 to Wisdom and Charisma, but -2 to Constitution. They're Medium Sized Humanoids who have Normal base speed, with a +1 Natural Armor bonus, two 1D4 damage Claw attacks, and Darkvision 60 feet. They also get a specific bonus trait based on what kind of Hag they are related to. Initially, Changelings only had Annis Hag, Green Hag and Sea Hag powers, but they received variants for each of those in the Advanced Race Guide and new powers for Blood Hag, Winter Hag, Mute Hag and Storm Hag mothers in Inner Sea Races.
- Annis Hag Changelings can have either Hulking Changeling (+1 racial bonus on melee damage rolls) or Mist Child (+5% to miss chance of attacks made against her whilst she has concealment or total concealment).
- Green Hag Changelings can have either Green Widow (+2 racial bonus on Bluff checks if the target is sexually attracted to her) or Object of Desire (+1 to caster level for Charm Person and Charm Monster).
- Sea Hag Changelings can have either Sea Lungs (can hold breath for (three times Constitution) rounds before starting to drown) or Ocean's Daughter (+1 trait bonus on Swim checks, automatically succeeds at Swim checks made to avoid nonlethal damage from swimming).
- Blood Hag Changelings receive Crawling Skin, which gives them a +2 racial bonus to Disguise checks made to imitate a specific individual.
- Mute Hag Changelings receive Dead Eyes, which gives them one eye that is a solid dead black in color but which also gives them a +2 bonus on all saves against gaze attacks and visual illusions.
- Winter Hag Changelings receive Frostlaced Flesh, which gives them a +2 racial bonus on all saving throws versus cold effects and means they leave no tracks when moving through snow.
- Storm Hag Changelings receive Into The Wind, a +1 racial bonus on ranged damage rolls.
There are no Night Hag Changelings; as Evil Outsiders, Night Hags spawn Half-Fiends in Pathfinder.
Inner Sea Races presents two alternative racial traits for changelings; Witchborn, which lets them change out the +2 Wisdom for +2 Intelligence, and Hag Magic, which lets them choose a single 1st level or 0 level spell from the Witch list in place of their Natural Armor Bonus. The chosen spell becomes a spell-like ability (caster level equal to the changeling's character level) usable either 1/day (1st level) or 3/day (0 level).
|The Races of Pathfinder|
|Player's Handbook:||Dwarf - Elf - Gnome - Half-Elf - Half-Orc - Halfling - Human|
| Aasimar - Catfolk - Changeling - Dhampir - Duergar |
Drow - Fetchling - Gillman - Goblin - Grippli - Hobgoblin
Ifrit - Kitsune - Kobold - Merfolk - Nagaji - Orc - Oread
Ratfolk - Samsaran - Strix - Suli - Svirfneblin - Sylph
Tengu - Tiefling - Undine - Vanara - Vishkanya - Wayang
|Bestiaries:|| Android - Astomoi - Caligni - Deep One Hybrid - Gathlain |
Gnoll - Kasatha - Munavri - Naiad - Orang-Pendak
Reptoid - Rougarou - Shabti - Trox - Yaddithian
|Adventure Paths:||Being of Ib - Kuru|
|Inner Sea Races:|| Ghoran - Monkey Goblin - Lashunta - Skinwalker |
Syrinx - Triaxian - Wyrwood - Wyvaran
|Ultimate Wilderness:||Vine Leshy|
|Blood of the Sea:||Adaro - Cecaelia - Grindylow - Locathah - Sahuagin - Triton|
|Planar Adventures:||Aphorite - Duskwalker - Ganzi|
Magic: The Gathering
A Static key word ability found on only on shape shifters. The ability makes the card it is on every creature type. This is a characteristic defining ability, so it applies no-matter what zone the card is in. . The history of the mechanic can be read here 
There are actually a few non-creature cards with the subtype thanks to the briefly used Tribal supertype. Having every creature subtype is mostly only used for Haakon, Stormgald Scourge to play them from infinitely so long as you have the mana.
World of Darkness
White Wolf's World of Darkness lines have a 'Changeling' game as part of their original series, their LARP series, and the rebooted series. They are more-or-less on opposite ends of the tonal spectrum.
Changeling: The Dreaming is for faeries that are born into mortal bodies and dealing with their dual natures. Characters live in two worlds simultaneously: the mundane one we're all familiar with, overlapped by a world we might call make-believe but is very real for them, and the two worlds can affect each other. There is always a tragedy with World of Darkness games; for Changelings, as they grow up the magical world becomes less and less real to them, until eventually they "grow up" and forget their natures as faeries. All Changelings eventually succumb to mundanity, living the rest of their lives locked out of their potential for magic. The different character classes are types of faerie: sidhe, troll, pooka, silkie, etc.
Changeling: The Lost is for mortals that were kidnapped by fae, and managed to escape back to the real world but are forever scarred by the experience. Resuming their old lives is made difficult by time-dilation effects during their imprisonment, and possibly by the simulacrum or impostor that replaced them when they were kidnapped (who is usually either wholly convinced that they are the person who they were meant to replace and will probably react violently when informed otherwise, or a complete psychopath, that makes the changelings not want to resume said lives). Characters are a little nonhuman themselves since they've spent time among fae, usually as toys or slaves while being subjected to positively Lovecraftian tortures, which also have a habit of mutating them into monstrosities and oddies, ranging from living ice sculptures to literally heartless hulks, and have a hard time acting "normal"; in some cases, they can end up turning into full-fledged fae themselves. Even if they do manage to adjust reasonably well, there's always the issue of their old masters trying to recapture them. Character classes are damned complex with 39 Seemings+Kiths, 12 Courts, 9 optional Entitlements, 28 Contracts, and that's just what's in the core book.
Dresden Files RPG
In the Dresdenverse, Changelings are those humans lucky (or more likely unlucky) enough to have a Fae for one of their parents. Their non-human traits manifest at puberty, giving them anything from super strength to really good looks. Unfortunately for them, they also get a partially Fae mind too, meaning that they have to deal with what can only be described as mental illness, ranging from dementia, to psychotic urges, to anger issues. Even more unfortunately for them, they are considered to be members of whatever Fae nation their parent belonged to, which means if they're Winterfae changelings, they're beholden to sadistic killers and emotionless sociopaths. And Summerfae changelings don't really get a good deal either, they have to answer to orwellian Knight-Templars who often place emotion and passion above reason and common sense. Even if their Fae parent genuinely loves and cares for them, chances are, they believe in using tough love to prepare them for the harshness of the world, which amounts to putting them in life threatening situations on a regular basis.
Changelings can choose to be either fully human (sane, mortal, and powerless) or fully Fae (lacking some of the qualities that made them human before, but powerful and functionally immortal). Until they Choose, they remain Changelings, trapped between two worlds. One would think that the Choice to remain human would be easy but Changelings always feel their Fae natures calling to them, promising a more fulfilling and rewarding existence, all they need to do is throw away silly things like humanity, Free Will, romance that doesn't involve the compulsive urge to manipulate and/or murderfuck their loved ones, and the ability to tell lies.
See The Changeling