Metallic Dragons are one of the many specific breeds of dragon native to Dungeons & Dragons and Pathfinder. They are one of the two most iconic family groupings of D&D dragons, alongside the Chromatic Dragons. Lesser D&D dragon families have include the Catastrophic Dragons, Gem Dragons, Oriental Dragons (or "Imperial Dragons", for Pathfinder), Planar Dragons, and Epic Dragons.
In an expansion of the alignment mechanic, chromatics helped define what our frenemies on TVTropes calls "Color Coded For Your Convenience", where the general color-scheme of a dragon immediately helps you figure out what they are as a general rule.
Metallic Dragons are based on the colors of various metals and are traditionally portrayed as the "goodly dragons". There has been some contention about this over the years; as most metallics are "noble" metals like copper, gold and silver, whereas "baser" metal-based dragons were given a tentative life as the
Ferrus Ferrous Dragon sub-family in Dragon Magazine#170 and #356, for AD&D and 3rd edition respectively. In 4th edition, meanwhile, WoTC pointed out that "Always Good" creatures are kind of less than useful to DMs, since most parties won't play evil campaigns and there's only so many times you can use "the super-intelligent and ultra-wise being of good is misguided/a dick/incorrect/mistaken" before it feels hollow. So they changed the default Metallic alignment to Unaligned and portrayed them as usually meaning well, but being arrogant and inhuman and so having lots of believable reasons to get in the way of heroes without being the malicious monsters of their chromatic kin.
The patron god of all Metallic Dragons is Bahamut, whose status as being based on platinum makes him unique.
The original five
The iconic quintet of Metallic Dragons in D&D goes: Brass, Bronze, Copper, Silver and Gold, ascending in power (at least from copper through silver to gold). Unfortunately, this quintet hasn't entirely sunk into the fantasy consciousness the same way that the Red/Blue/Black/White/Green quintet has. At the very least, it rather throws off the theme when three are noble metals and two are copper alloys, though at least all five metals have historically been used as coins so the theme does make sense with some thought.
Chatterboxes. Brass Dragons prefer hot dry areas like deserts. Brass Dragons like nothing more than sunbathing in a good spot and prattling with a stranger for hours on end, even if they have to bury them up to the neck in sand to do so. Brass Dragons have big head crests, and kind of look like dinosaurs. They are the everyman/socialite of metallic dragons, talking their way out of trouble and building networks of contacts and informants. If you want to include a dragon in your campaign, but aren't sure how, throw in a Brass Dragon questgiver.
Honorable champions of order, but prone to Lawful Stupid. Also live on coasts, makes good fluff for Dragonborn marine.
The Copper Dragons are most known for being the practical jokers of the Metallic Dragons.
Regal and imperious, Gold Dragons traditionally hold the position of being the mightiest of the Metallic Dragons, although that has been shaken up before - most notably in 4th edition. Originally, Gold Dragons were designed to more of an "eastern dragon" body-structure, with long, serpentine, wingless bodies and flowing mustaches. Their bodies have changed to match the standard "western dragon" archetype, but the pseudo-mustaches seem here to stay.
While the copper, silver, and gold dragons all have the ability to polymorph themselves into humanoids and
have sex with live among mortals, silver dragons are noted for being by far the most fond of doing this. If you come across a creature with the half-dragon template, it probably got its dragon half from a silver dragon. They're basically the furries of the dragon world. There's even an entire human subrace, the Silverbrow humans, who are the result of a LOT of human-on-silver-dragon action, which you can't say about any of the other types of dragons. What we're trying to say is that they're sluts.
Other metallic dragons
Whilst a few additions to the Metallic family tree have appeared here and there - mostly in the Forgotten Realms - it was 4th Edition that took the stance of shaking up some sacred cows. Pointing out that 1: Bronze and Brass threw off the Noble Metal theme of the "core" metallics, and 2: Bronze, Brass and Copper have always been hard for anyone except the most fanatically invested neckbeard to tell apart, they removed both Bronze and Brass, delegating them to a later appearance in the Metallic version of the Draconomicon and replacing them with new Adamantine and Iron Dragon species.
Adamantine Dragons are heavily-armored Metallics native to the Underdark, who may arguably be more powerful than even Gold Dragons, with distinctive beak-like snouts and a breath weapon that deals thunder damage. They are essentially the Metallic analogue to the Purple Dragon, favoring the Underdark as their domicile of choice. These dragons were entirely unique to 4e; AD&D had featured an Adamantite Dragon, but that was a Planar Dragon native to the Twin Paradises of Bytopia.
Like the Iron Dragon, Cobalt Dragons are a Ferrous Dragon breed that got promoted to Metallic in 4th edition, with quite a lot of differences. The 4e Cobalt Dragon is a grim, vicious, possessive, powerfully built Metallic Dragon that looks a lot like a flying bear. They absolutely hate the heat, establishing tyrannical dominions in colder regions where they are comfortable. Possessed of an exorbitant amount of martial pride, Cobalt Dragons prefer valuable arms and armor for their hordes, and relish commanding skilled, war-like minions.
Iron Dragons originated in Dragon Magazine #170 as members of the Ferrous Dragon family - see that page to learn about how they looked there. As a core member of the Metallics, 4e's Iron Dragons are essentially a Metallic analogue to the White Dragon, being simple-minded, brutish and violently aggressive dragons who rely on electromagnetic attacks and thick scales to fight their foes.
Fickle, whimsical and highly chaotic, the Mercury Dragon is an obscure beastie that is native to the Forgotten Realms, having first appeared in the original Faerun-set Draconomicon before hitting the big leagues with a place in the AD&D Monstrous Manual, followed by 3rd edition's "Dragons of Faerun" and then the 4e Metallic Draconomicon. Traditional Mercury Dragons use laser beam breath weapons and reflection-based attacks. The 4e version is a superb shapeshifter that might as well be made of living metal - picture a draconic T-1000 Terminator - and wields a poisonous breath weapon.
Unique to 4th edition, Mithral Dragons are the elite of the Metallic Dragon family tree, native to the Astral Sea and wielding a diverse array of powers that allow them to bend space and time to their will. In this, they could be argued as a spiritual inheritor to the original Adamantite Dragon, but their arrogant self-righteousness and desire to change the world - regardless of what the world thinks - is perhaps closer to the 4e rendition of Couatls.
Entirely unique to 4th edition, Orium Dragons are red-gold colored dragons, with serpentine heads, necks and tails mounted on a rather feline body frame. Based on "orium", WoTC's trademarkable name for "Orichalcum", they are obsessive historians who seek out ancient ruins to maintain, preserve and restore. Their unique breath weapon is a gout of toxic vapor, which then coalesces into a snake-like construct that keeps on fighting on its own after being exhaled.
The Steel Dragon has a rather complex history, and is arguably not a "true" metallic dragon, having no asserted ties to that category prior to 4th edition.
Steel dragons first showed up in Advanced Dungeons & Dragons as a draconic breed unique to the setting of Greyhawk. This, combinined with their prominent presence in the City of Greyhawk, led to their alternative nickname of "Greyhawk Dragons". They made their debut in the Greyhawk Appendix for the Monstrous Compendium, and were subsequently reprinted in 1993's Monstrous Manual, albeit with a fancy new colored art piece.
The confusion stems from the fact that another Steel Dragon debuted in 2e as well; the Faerunian Steel Dragon, in the original Draconomicon. Whilst their basic personalities (and thus fluff) were identical, save for an expansion on habitat/society lore, they differed in their abilities.
Oerthian steel dragons exhale a cloud of short-lived but lethally poisonous gas, whilst Faerunian ones breathe a stupefying gas instead. Faerunian dragons have tougher scales than their Oerthian counterparts (AC is 2 points lower at all stages), and their magic resistance is superior; both dragons are flat-out immune to all Wizard spells of 1st to 4th level, but Faerunian steels have a 75% magic resistance on top of that. Finally, their spell-like abilities gained from the Young to Old stages of their lives are different:
- Greyhawk Dragons gain access to Cantrip 2/day, Friends 1/day, Charm Person 3/day, Suggestion 1/day, Enthrall 1/day
- Faerunian Steel Dragons gain access to Detect Lie 3/day, Charm Person 3/day, Suggestion 3/day, Antipathy/Sympathy 2/day, Imbue With Spell Ability 1/day
And for added insult, the Greyhawk Dragon casts these abilities as if they were 8th level, and the Faerunian Dragon casts them as if they were 11th level!
The fact that these dragons was so close did not go unacknowledged. The Draconomicon itself states that two races are "almost certainly related", but insists they're not the exact same breed. This was doubled down upon in issue #206 of Dragon Magazine, where the Half-Dragon expansion portrayed Half-Greyhawk Dragon and Half-Steel Dragon as being two different things.
Dungeons & Dragons 3rd Edition played with this idea. The Greyhawk Dragon made a 3.5 debut in the Creature Catalog IV article in Dragon #339; the Faerunian Steel Dragon would show up in Dragons of Faerun. Their flavor text was almost identical, but with the setting specifics swapped out - the 3.5 Greyhawk dragon's fluff explicitly states that Steel Dragons are their descendants born from dragons that traveled off of Oerth. Both species now had the same breath weapon; a twofer that could be deployed as either a line of acid or as a cone of poisonous (Constitution damage) gas. However, Faerunian Steel dragons lack the Greyhawk Dragon's Minor Arcane Shield Trait, have the Alternate Form supernatural ability instead of the Greyhawk Dragon's Polymorph Self 5/day spell-like ability, and are true dragon spellcasters who can also cast clerical spells, especially those of the Knowledge and Trickery Cleric Domains. The DoF version got a lot of attention because, unlike most monsters, it was actually pretty viable as a player character with the optional rule for LA buyoff in place.
Finally, Dungeons & Dragons 4th Edition came along and decided that this was a pointless headache. It formally made Steel Dragons a part of the Metallic Dragon family, and got rid of the setting specific versions.
Despite this long and spiralling path, the Steel Dragon's personality has remained remarkably consistent throughout the editions. Regardless of which version you look at, all steel dragons find humanoids fascinating; they are the most "civilized" of the dragon breeds, using their ability to assume humanoid form to integrate neatly into humanoid societies. Whilst they still pursue the typical draconic pursuit of wealth, they also legitimately enjoy being part of their communities, and their draw to the position of scholars and sages is as much an excuse to learn more about the people with whom they live as it is to provide an outlet for draconic pride.
4th edition did make a slight change to this characterization when it added a certain chaotic tinge to the race; the 4e Steel Dragon is a huge believer in personal liberty, and cannot abide tyranny. This makes them surprisingly bitter enemies of some breeds of Metallics; the readiness with which Bronze and Gold Dragons will take absolute authority and justify it as "I know best" incenses the 4e version of Steel Dragons, who do their best to take these "benevolent tyrants" down a few pegs.