Vodnik/Vodyanoi/Vodyanoy

The Vodnik is a creature from Slavic mythology. They are sometimes thought to be the ghost of a drowned child and they may take the form of a living child to lure other children in to meet the same fate. When the child approaches the deep or rapidly moving water, the Vodnik holds them down and drowns them. They may also take the form of a fish, a fish/human hybrid, a coin in the water, a floating log, a large fish, a fiery-eyed monster, a human-faced seal, a giant frog, or an old man with green hair and beard. They lurk at the bottom of bodies of water and drag people under. At the new moon, they are young, and they become old over the month, becoming rejuvenated at the following new moon.

Citations:

Bane, Theresa. Encyclopedia of Fairies in World Folklore and Mythology. United States, McFarland, Incorporated, Publishers, 2013.

Maberry, Jonathan. Vampire Universe: The Dark World of Supernatural Beings That Haunt Us, Hunt Us, and Hunger for Us. United States, Kensington Publishing Corporation, 2006.