Nixie (folklore)
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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
Merman
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A merman, the male counterpart of the mythical female mermaid, is a legendary creature which is human from the waist up and fish-like from the waist down, but may assume normal human shape. Sometimes mermen are described as hideous and other times as handsome.
Triton with a nymph
Triton ―Schott's Physica Curiosa (1697).
The merman (1911) by John Bauer