The Nixie, Nixy, Nix, Näcken, Nicor, Nøkk, or Nøkken are humanoid, and often shapeshifting water spirits in Germanic mythology and folklore.
Nøkken by Theodor Kittelsen, 1904
Näckens polska by Bror Hjorth
Näcken ("The Water Sprite") by Ernst Josephson, 1884
The Neck as a brook horse by Theodor Kittelsen, a depiction of the Neck as a white horse
A water spirit is a kind of supernatural being found in the folklore of many cultures:
Water Spirit mask from the Igbo people (Brooklyn Museum)