In Slavic mythology, vodyanoy is a water spirit. In Czech and Slovak fairy tales, it is called vodník, and it is considered to be the equivalent creature as the Wassermann or nix of German fairy tales. In Ukrainian fairy tales, it is called “водяник“ (vodyanyk).
Vodyanoy by Ivan Bilibin, 1934
Typical projection of vodník in Czech or Slovak folklore
Czech Vodník in Peklo u Nové Město nad Metují
A water spirit is a kind of supernatural being found in the folklore of many cultures:
Water Spirit mask from the Igbo people (Brooklyn Museum)