An insect

Probably caused by radiation, an insectoid male only race appeared, equipped with an ovipositor that was slimy and perfect for breeding. The way they reproduced was via implanting their eggs inside the ass of their unfortunate breeder, regardless of gender. They achieve this by fucking human butts while holding their hips with their own pincers. The human becomes a queen and starts experimenting growth in its belly initially until it accumulates enough nutrients to be able to lay the eggs. The larvae feed on that and grow up to somewhat big and eager. They feed on their fluids for several months until they grow enough to breed with their queens. The adults usually keep their queens well feed with a jelly like substance that comes from their penis.

However, humans and these insects coexist in harmony. The insectoids are very sentient and are completely harmless as long as their compound eyes don't see an unprotected ass, because that makes them think with their ovipositors. This causes humans to just wear vintage pantie that cover the entire butt to avoid getting and pregnant.

So you are surprised when your roommate is one of these creatures, smaller yet stronger than you. All you have to do is to keep your anus hidden...