Mongrelmen are a Dungeons & Dragons race that... well, basically they exist to poke fun at the idea of interspecies breeding.
It's like this: you know how humans can crossbreed with just about anything, right? Hence the existence of half-elfs, half-orcs, half-dwarves, half-Ogres, half-Goblins, half-Gnomes, half-Gnolls, half-kender... you get the idea. Plus, many humanoid races could crossbreed with each other; orc/ogres, orc/goblins, and so forth. Well, what happens if you keep mixing species together, generation after generation?
According to Advanced Dungeons & Dragons, you end up with a bunch of hideously deformed monsters whose bodies are quite literally patchwork amalgamations of various different traits, both humanoid and animal. Skin that's tapestry of different traits (hairy patches, scaly patches, feather clumps, smooth, warty, and different shades), mismatched limbs and organs, mongrelmen are literally nothing more than an asymmetrical assemblage of random body parts - not helped by the fact that, at the time, "monstrous humanoid" wasn't seperate to humanoid, so you could have humans successfully crossbreeding with bullywugs, lizardfolk, gnolls and minotaurs, among others. Needless to say, they're best known for being hideous and ugly; most "goodly" humanoids tend to shun them, whilst most evil ones tend to enslave them.
Despite this, mongrelmen are generally a peaceful race, who mostly want to be left alone. Given their freedom, they establish camouflaged communities in the wilderness and live simple agrarian lives, avoiding contact with other races and simply keeping to themselves.
Mongrelmen have undergone some lore changes over the editions.
Firstly, in their "Ecology of" article in Dragon Magazine #242, they were changed from being "the bastard sons & daughters of many generations of cross-species breeding" to the unwitting creations of a wizard who wanted to engineer a shapeshifting servitor species - sort of a tame doppelganger. He succeeded in the form of the Infiltrators, who could consume samples of flesh from humanoid species in order to "store" that genetic material, which would enable them to shift into that species at will. Unfortunately, his creations weren't true-breeding, and instead their offspring were hideous amalgamations of traits randomly selected from the various species assimilated by their parents. Worse for the poor Mongrelmen, they have corrupted the memories of their ancient wizardly creator into revering him as a god, and continue their Infiltrator ancestors' practice of Feasting (consuming scavenged flesh from humanoids), which only continues to warp and pollute their bloodline with random traits.
Secondly, in third edition, Mongrelmen (renamed to the more politically correct Mongrelfolk) reappear in Races of Destiny for 3rd edition, in which they have become a lot tamer. Whilst still the result of multiple generations of cross-species breeding, the end-result is not the hideous patchwork of editions past, but a harmonious blending of humanoid traits that can readily pass for any of its parent species, with the proper preparations. Indeed, they are instantly accepted as just another member of a humanoid race when they meet one of said races... just not the same race as the viewer. So, for example, a dwarf might see a mongrelfolk as an unusually short and broad-shouldered elf, whilst an elf would instead see him as an unusually tall and slender dwarf.
In fifth edition, after returning in the Curse of Strahd adventure, they went back to the "hideous amalgamation of human and random animal traits" lore, but were closer to Broken Ones in terms of fluff, being created by magical experiments.
Mongrelmen have had playable stats in the Complete Book of Humanoids and Dragon Magazine #242 for AD&D, and in Races of Destiny for 3.5.
|Dungeons & Dragons 2nd Edition Races|
|Core:||Dwarf - Elf - Gnome - Half-Elf - Half-Orc - Halfling - Human|
|Dark Sun:||Aarakocra - Half-Giant - Mul - Pterran - Thri-kreen|
|Dragonlance:||Draconian - Irda - Kender - Minotaur|
|Mystara:|| Aranea - Ee'ar - Enduk - Lizardfolk (Cayma - Gurrash - Shazak) |
Lupin - Manscorpion - Phanaton - Rakasta - Tortle - Wallara
|Oriental Adventures:||Korobokuru - Hengeyokai - Spirit Folk|
|Planescape:||Aasimar - Bariaur - Genasi - Githyanki - Githzerai - Modron - Tiefling|
|Spelljammer:||Dracon - Giff - Grommam - Hadozee - Hurwaeti - Rastipede - Scro - Xixchil|
|Ravenloft:||Broken One - Flesh Golem - Half-Vistani - Therianthrope|
Book of X:
| Alaghi - Beastman - Bugbear - Bullywug - Centaur - Duergar |
Fremlin - Firbolg - Flind - Gnoll - Goblin - Half-Ogre - Hobgoblin
Kobold - Mongrelfolk - Ogre - Ogre Mage - Orc - Pixie
Satyr - Saurial - Svirfneblin - Swanmay - Voadkyn - Wemic
|Dragon Magazine:||Half-Dryad - Half-Satyr - Uldra - Xvart|