I tend to prefer the term "Undine" more than Mermaid or Merfolk... I don't know why... but for some reason "Merman, Mermaid and merfolk" sounds so childish or something. It sounds like something some silly human would come up with to call something that he didn't even talk to.
do you honestly think that such creatures would call themselves that? I don't think so.
But yeah, the Undine myth is a truly fantasicing one. they take on so many different forms depending on which nationality or region. and for each nationality, they go by different names too.
My favorite type is the selkies though... they have like a hundred of far more intersting stories out of all those mythologies.
Selkie--(also known as silkies or selchies) are creatures found in Faroese, Icelandic, Irish, and Scottish mythology. the top is like that of a human, but the bottom part is like a seal's. apparently they have those seal tails because of this magical belt they have on all the time.. and when they take this belt off their tails magically become human legs, and they're able to take walks alongside the coasts. Sometimes they will even take human lovers and live on land for a while... and the humans wouldn't even know they were a selkie.
http://en.wikipedia.org/wiki/Selkie