This is a really freaky one... The Candiru...

Candiru or (or willie) fish, refers to parasitic freshwater catfish of a number of genera in the family Trichomycteridae. They are found in the Amazon River and have a reputation among the natives as the most feared fish in its waters, even over the piranha. They are eel-shaped and translucent, making them almost impossible to see in the water.

This fish is also known to attack humans and animals and swim into an orifice (the vagina, anus, or even the penis—and deep into the urethra). Because of spines protruding from the fish, it is almost impossible to remove except through surgery. Though there have been documented candiru attacks on humans, there is no evidence the fish can survive once inside a human.

Wikipedia links:
ENG: Candiru or (or willie) fish
NL: Candiru or (or willie) fish