xXxStrawberry of LovexXx
In some myths Mermaids are just playful creatures, but in others, they are horrible creatures that want to drown men, and women who are prettier than they are.
The ones about drowing people, yeah, they're sirens, people often mistake the two because they both have fish tails, though sirens can really be half of any animal, just that the fish-tailed ones are more common in ancients myths. And un-like mermaids the Siren's can change from animal to half-animal to human, at will. Though some myths suggest that, kind of like the werewolf, they only change at night into their animal forms.
As for mermaids, they frankly can be nasty at times but most often than not they're not, though I think it would be ten times more interesting if they were all nasty. Sirens, they'd hold my attention far longer than any mer-person would.
Also both seem to always be described as very beautiful, the only difference is that the sirens always have beautiful melodic voices that draw people to them so that they can drown them.