In Slavic myth and folklore, the vodianoi is the unpredictable, dangerous king or spirit of water, particularly of freshwater.
As a water spirit, the vodianoi needed no clothes. He generally was seen by humans as a being that was half fish and half human, or as an old man covered in scales and mud. His hair and beard were green, and his hands were webbed.
When he chose to have a tail, he looked something like a heavyset merman. When he decided to take a two-legged shape, his long toes helped him to propel himself underwater.
The vodianoi was not actively evil, but he lacked all concept of human morality. He sometimes willfully drowned humans out of sheer dislike, but more often he dragged them underwater to provide entertainment for himself and his wife, the vodianikha. Those humans who were foolish enough to bathe at twilight were at greatest risk of being snatched.
When he became truly angry with humanity, the vodianoi was said to cause floods that destroyed dams and mills. Wise millers and fishermen made him offerings to keep him docile. The vodianoi was considered to be wealthy due both to these offerings and to the bounty he had taken from sunken ships.
In the Christian era, the vodianoi was often confused with the Christian devil, as both were believed to live underwater and to look alike, except, of course, that the devil had horns.