Look... humans... they get around, alright? And sometimes, those couplings produce offspring that are a mix between the two parent's species. That's where a hybrid comes in. Skirmisher Publishing has released Monstrous Hybrids, a book primarily for Pathfinder that discusses three different hybrids and their abilities in the game.
From the website:
Presented here are three hybrid races, beings that cannot help but exhibit the dark side of their heritage. Beast-Kin are savage beings that are as much animal as human; Half-Ghouls are the spawn of humans and carrion-devouring undead; and Vardogers are a hybrid race of lesser shape-shifters descended from Doppelgangers.
This mini-sourcebook has been specifically designed for use with the Pathfinder Roleplaying Game but can be used with any OGL-based game pretty much as-is and easily adapted to any related or successor systems.