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In Slavic mythology, a vodyanoy (plural vodyanoyovia) is a male water spirit or Folk boggart in Czech tales. Vodnik (in folk fairy tales) is same creature like Wassermann or nix of German fairy tales.

He is said to appear as a naked old man with a greenish beard and long hair, with his body covered in algae and muck, usually covered in black fish scales. He has webbed paws instead of hands, a fish's tail, eyes that burn like red-hot coals. He usually rides along his river on a half-sunk log, making loud splashes. Consequently, he is often dubbed "grandfather" or "forefather" by the local people. Local drownings are said to be the work of the vodyanoy. He would either marry Rusalkas, or have them as servants.

In Russian folklore, they are variously described as a floating, moss-covered log with wings; an old man with a blue face, white beard, and green hair; an old man covered in scales or fur with huge paws, glowing eyes, horns, and a tail; or entirely as an enormous grotesque fish. In their humanoid form they would seem younger or older with the phases of the moon. The Vodyanoi was said to dwell in the very depths of the water, where it had either a beautiful illuminated palace that glowed on certain nights of the year, or to live in the very slime of the bottom with which it was sometimes covered. It was to be seen lurking in rivers, mill pools, and ponds, where it would lure any unwary humans to a horrible death in the water. This monster was a threat to all humans except the millers and fishermen. It would appear at night very frequently in the mill race, and to keep the Vodyanoi from doing harm, the millers would propitate him with a cockerel. Often in the past a drunken stranger passing through the water mills would also have been sent tumbling into the mill pool in place of the cockerel--and was then devoured by the waiting Vodyanoi.

Rose, C. Giants, Monsters, & Dragons: An Encyclopedia of Folklore, Legend, and Myth. Norton Publishing Company. 2001.