[the Platinum Poppy]

Hm, I googled around and it seems like "Näcken" is peculiar to Scandinavia. So although everyone here knows about him perhaps he's new to you.

This is a creature who usually looks like a really hot guy, and lives underwater in rivers and lakes. He plays the violin more beautifully than any human being. He'll sit perched on a little rock in the water, naked, playing his violin, to lure women to him. But if anyone is stupid enough to wade into the water to reach him she'll get drowned. I have no idea why he's supposed to lure women out to be drowned like that. But according to the myth he has some kind of connection with Satan, so it's probably out of pure evil.
He can also shapeshift into a pretty white pony. When he wants to kill little children he'll prance around at the beach in that form, and if anyone is stupid enough to get up on his back to ride him he'll run out into the water and drown the kid.

I guess there's a moral here. Don't ride unknown ponies, and don't run straight into the water to unknown naked men no matter how hot they are.