merman
A mythical sea creature which has the top half of a human male and the hind half (fins) of a fish. Less commonly known and often depicted less attractive than their female counterparts, though they've been looking more human and just as pretty as their female counterparts as time goes on.
Not to be confused with fishmen, which have a humanoid hind half but more fishlike characteristics throughout their bodies.
See also
This tag implicates monster_boy (learn more).