In Finnish mythology, a Näkki (Estonian: Näkk) is a water spirit that resides in murky pools, wells, docks, piers and under bridges that cross rivers.
Näkki is also called Vetehinen or Vesihiisi (water fey, see Hiisi. It is closely related to the Nix in Scandinavian folklore or Russian Vodyanoi.
Näkki is said to be very beautiful from the front, whereas 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.
Näkki 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.