In The Wheel of Time, Trollocs are synthetic race, created by Aginor during The War of Power (also known as The War of Shadow) to serve as footmen in Dark One's army. They were created by breeding men with animals (apparently Aginor did know some genetic, I haven't heard of half-man-half-sheep no matter how hard farmers tried).
Trollocs have varying looks, what they have in common is just that they walk on two legs, have generally human-like form but various bodyparts are replaced with parts of other animals, like, hooves instead of legs, beak instead of nose etc. (now, that's some genetics, someone please tell me that actually is possible).
Trollocs are generally dumb (although there are exceptions, such as famous philosopher and linguist Narg, who found the Way of the Coconut), smelly, ugly and they like to kill. They like it so much they spend more time killing each other than humans, actually. Mysteriously, despite this enthusiasm for killing, a kid who hasn't wielded sword for 2 months is able to kill dozens of them easily. Suppose being a hero can do that.
Sometimes Trollocs spawning with each other produce something that is called Myrddraal.