Vodyanoy
Vodyanoy (dedushka vodyanoy) is the East Slavic male water spirit, ruler of rivers, lakes, and millponds. Appears as a naked, green-haired old man with a pot belly, covered in slime and weeds, or as a giant catfish/frog. Demands blood sacrifices (livestock, historically human) when mills are built. Helps fishermen who offer butter; drowns the disrespectful. Rides a wels catfish.