Candiru
These smaller species are known for being thought to invade and make use of a person's urethra. Even with reports as old as the 1800s, the first written case of the removal of a candiru from a person did not occur until 1997, and even that incident is not believed by everyone. The idea that the fish are attracted by urine appears to be not right. The fish actually hunt by sight and do not like urine at all.