But I don't want these to just be alternate species of hominid that are maybe just a little greener in complexion. Oh no. I wanna horror them up a bit. Jack, over at Tales of the Grotesque and Dungeonesque, did something similar a couple years back with a series on Demi-Humans in the World Between, and I'm trying hard not to simply ape him with these.
Orcs have relieved themselves of the pain of being men by making beasts of themselves. Having renounced faith and given in to blaming God for the Cataclysm, they've given into cruelty, to hate, to rage, and have made violence their only credo; all vestiges of human civilization have been shed. Their bodies reflect this beastly nature; some have goat horns, others boar tusks, still others paws like a bear's. Sometimes entire bandit camps undergo metamorphosis simultaneously, becoming a gang of orcs overnight. Many brigand groups includes orcs among their number, simply accepting that their former human comrades are now red-eyed howling savages.
Trolls, with their enormous, grasping hands and tenacious grip on life itself, were once men whose avarice knew no bounds; greedy barons who feasted while their subjects starved, priests who demanded greater tithes then their parishes could support - anyone who hurt or killed others in the pursuit of possession for possession's sake.
Goblins were once men who cringed and bent the knee to Chaos out of fear, rather than devotion. For their cowardly toadying, these men are reshaped into skulking, moon-eyed sycophants, easily-cowed creatures whose nature demands they serve the strongest presence available. As such, goblins are almost always found accompanying an orcish leader, an Ogre, Troll, or some form of demon.