It kinda treads on the line for the OP's request, but the Tsochar from Lords of Madness creep me out to no end.

Hive mind leech/snake things that climb inside your body and force you to do their bidding or suffer horrible agony, and can also decide keeping you alive isn't worth the trouble so they just eat your brain and drive your body around like an old volvo.

*shudders*