Quite similar to a hermaphrodite, a mermaphrodite is a merperson, with the genitilia of both a merman and a mermaid.
"So my ship gets wrecked at sea and I'm thinking, 'Alright! You're going to die.' But just then, out of nowhere, I get pulled to safety by this thing! Like a mermaid, but it sorta looked like Jamie Lee Curtis. When I noticed it's adam's apple I realized it must have been a mermaphrodite."
by Marc Sauve December 16, 2005