Warning, some of this material may be disturbing.
One idea that I have seen little development in is non-sapient mutant humans. They have lost their ability to reason due to mutation and have become animals, their niche determined by their physiology and the actions of intelligent species. Here are a few ideas to creep players out.
Predators. Watching a naked human kill and eat something with their hands and teeth are distrurbing enough but adding in mutations and the ML can start adding in body horror as well. Fortunately as they are animals, these humans can be driven off with fire, loud sounds and similar things. Of course that may very well enrage them or draw more to the party.
Parasites. Yes, tiny humans attached to some other creature (including plants) like leeches or ticks. Read up how those animals feed and take notes. When the swarm of parasitic humans detach from their dead host and attempt to latch on to the PCs, expect some sounds of disgust from the players.
Livestock. Would it be worse to have mutant cattle or pure humans raising non-sapient humans for leather and meat as well as pulling plows? If they are clothed by their owners (not masters, they aren't slaves), one might not even pick up on their lack of reason at first glance.
Surrogates. Some mutant humans are infertile within their own genotype, they are forced to reproduce with other humans. Some pay to have children and others use these non-sapient humans as brood mares or studs. A form of bestality that seems so wrong yet can you blame the people who can't have children otherwise?
And no picture, I creeped myself out enough, thank you very much.
