Näkki

In Finnish mythology, a Näkki (Estonian: Näkk) is a Neck, a shapeshifting water spirit who usually appears in human form, that resides in murky pools, wells, docks, piers and under bridges that cross rivers.

He is principally known for pulling young children into the depths, if they lean over bridge railings, docks or otherwise look into water surfaces to see their own reflection and touch the water. Näkki is a fine example of a spirit enlisted by parents to guide children away from unsafe practices.

According to Nordic mythology, during Midsummer's night, Näkki rises from the water to dance in the middle of the celebrating people.

It is also said that although Näkki is very beautiful from the front, his backside is hairy and extremely ugly. Other stories tell that a Näkki is an ugly "fishman" which can at will turn itself into a beautiful woman who either is extremely voluptuous or has three breasts or alternatively into a silvery fish, horse or a hound, which are only ways to lure his unwary prey to the water. Näkki is also called Vetehinen or Vesihiisi (water fey, see Hiisi).

...
Wikipedia