Vodianoi
The vodianoi is a male water spirit of Slavic origin. The Czech and Slovak equivalent is called a vodník while Polish is a wodnik. A South Slavic equivalent is vodenjak. He is viewed to be particularly malevolent, existing almost exclusively to drown swimmers who have angered him by their boldness. Reports of his appearance vary; some tales define him as a naked old man, bloated and hairy, covered in slime, covered in scales, or simply as an old peasant with a red shirt and beard. He is also reported to have the ability to transform into a fish.
The vodianoi lives in deep pools, often by a mill, and is said to be the spirit of unclean male dead (this definition includes those who have committed suicide, unbaptized children, and those who die without last rites). As previously stated, the vodianoi would drown those who angered him with boasts or insults. However, there was no certain protection, as the spirit was particularly capricious. Peasants feared the vodianoi and would often attempt to get rid of the spirit or, failing that, appease him.
The only people who were generally safe from the vodianoi's anger were millers and fishermen. Millers in particular were viewed to be so close to the vodianoi that they often became seen as sorcerous figures. This may be influenced by the belief that millers yearly drown a drunk passerby as an offering to the vodianoi. Fishermen were somewhat less suspect, offering only the first of their catch with an incantation. If a vodianoi favored a fisherman he would herd fish into the nets.
Read more about this topic: Supernatural Beings In Slavic Folklore