Ok, to date, all the paraites I've mentioned are invertebrates. So, to be fair, I'm going to make the next two weeks parasites that are vertebrates - after all, it's not like having a backbone makes you somehow a nicer animal.
Take the pearlfish.
I first heard about the pearlfish in my freshman year of college during an Invert Bio course, when it became the brunt of nerdy bio jokes. Technically, by just calling it a "pearlfish", I'm not being very specific - pearlfish refers to any member of the Carapidae family, a group of 6" long eel-looking fish. Most, in general, just happen to live in another species without harming anyone. They simply make the insides of another animal their home. Many, for example, live in sea cucumber cloacas - or, for those less invert-bio inclined, up a sea cucumber's ass. Seriously. They leave to go find food during the evening and return to the safe, predator-free home of their best friend's butt during the daytime. Hence the jokes... I think you see where our young, impressionable minds went.
That is, except for the ones in the genus Encheliophis - those ones are true parasites.
The parasitic pearlfish, to put a spin on parasitism, are vertebrates that parasitize invertebrates, usually Echinoderms like sea cucumbers and sea stars. Like all other pearlfish, they make the insides of another animal their home. It's like living in a gingerbread house - how can you resist the temptation to take a bite? There is all kinds of food to eat! Gonads, gills, you name it - a lovely meat buffet, all from the comforts of home. So the pearlfish in this genus somehow decided to take advantage of it.
Amazingly, sea cucumbers have the ability to completely expell their entire internal organs when provoked - usually as a way of avoiding predation. Yet, somehow, they don't do so when little pearlfishes swim in their anus' to get comfy. How come the pearlfish doesn't trigger the host's defenses? No one knows.
Anyhow, it seems sci-fi enough to me to have an organism living in your backside, although, to be fair, we have all kinds of digestive system parasites. Most try to avoid the very tail end, though - the stomach and the upper intestines are far more nutrient-rich, and most are microscopic or at least thin. Let's just hope nothing bigger than the parasites we have ever gets the nerve to try and act like a pearlfish in humans... I mean, we're talking a 6" fish. That would not be pretty.