MERMAID

A hot babe from head to waist, and a scaly fish from the waist to the tip of her tail. Mermaids like to sit on rocks and use either a) their enchanting sirens' songs, or b) the tried-and-true tactic of looking in a mirror and combing their green hair while they display their huge tits in order to lure human men (and especially sailors and pirates and stuff) to make love to them.
http://www.fateefate.it/sirene/pages/storie/storie.htm

Note that the mermen and merwomen do not do this, though young mermen have been known to kidnap human women and turn them into mermaids.
Mermaids are described as either mortal (with or without a soul) or fairies, so they may or may not command powerful magic. Some can see the future, turn their tails into hot legs, call up storms, sink ships with tidal waves, and do other cool stuff. Others can sing... and that's about it.
Mermaids have a very seductive nature, and almost always make love with sailors. No one knows they do this, but there is a theory that they are simply attempting to produce more of their species. When turned down on an offer of sex, though, the mermaid will be thrown into a fit of rage and slay the sailor who refused her pleasure.