The adorable Leucochloridium paradoxum - everyone's favourite mind-bending parasite. Parasites that manipulate the behaviour of their host always get one of those "Wow, freaky nature huh?"-reaction from the general public. Then they move on, taking note of that story to save for a cocktail party or something. However, for those of us in the know, the less palatable truth is that parasites that take control of their hosts appears to be the norm rather than the exception.
Leucochloridium paradoxum is a digenean trematode which uses a land snail as both their first intermediate host (where they undergo clonal multiplication) and second intermediate host (the means to reach the definitive host - where it can get up to hanky panky with other worms of its own species). Our intrepid parasite first castrates the unfortunate snails, leaving intact only the organs necessary to keep it alive, then takes control of its behaviour to try and convince a bird that it is in fact a pair of tasty caterpillars.
Here's a fun fact - there are over 15000 described species of digenean trematodes (with many, many more undescribed species) and they all (except for half a dozen or so species) use either a gastropod (e.g: a snail) or a bivalve (e.g: a clam) obligately as first intermediate host in their life-cycles. And all of them, all FIFTEEN THOUSAND of them, zombifies their molluscan first host completely - at least in body, if not in their mind as well.
You think you are looking at a snail? Hah! You're looking at a worm in snail's clothing, a parasite *wearing* a snail.
Body-snatching monsters in fiction holds no horror for me - they exist in reality, and I confront them on a daily basis - after all, it's my day job.