Candiru (Vandellia cirrhosa)

Found in the rivers of the Amazon, the species of parasitic pencil catfish is infamous for entering the urethra of humans urinating underwater, probably mistaking the urine stream for exhaust water exiting the gills of fish. Their normal hosts are other fish to which they attach themselves to the aortal arteries from within the gill chamber and ingest the blood of the host.

Image source 1, 2

(Source: ichthyologist)