Nickar

In Teutonic mythology, Nicor are malignant water monsters who drown people. They are related to the Nixie, which are kind, loving male water spirits like the Stromkarls and the Necks. The female water spirits are called Undine. These lesser water divinities will sometimes leave the water to attend village dances. They have fish tails and often sit by the water combing each other’s long green or blonde hair, playing the harp, or singing.

Dictionnaire Infernal – Collin de Plancy (1863) (paraphrased)
According to Scandinavian mythology, Odin takes on the name of Nickar or Hnickar when he acts as a destroyer. Under this name, he appears as a kelpie, the devil-horse of the Scots, & frequents the lakes & rivers of Scandinavia where he causes tempests, hurricanes, & hailstorms. On the Isle of Rugen, he likes to torment fisherman, upset their boats, and throw them almost to the tops of the highest fir trees. Nickar are descendants of the mermen and merwomen, the nixies of the Teutons, the most famous being the nymphs of the Elbe and of the Gaal. Before Christianity was adopted, the Saxons adored a feminine divinity, whose temple lies in Magdebourg or Megdeburch (city of the young lady). She would appear with a basket under her arm, graceful & proper, and at first glance, one could mistake her for the daughter of a good bourgeois. However, a small corner of her apron always remained wet, as a reminder of her aquatic origin. The English sailors are known call the devil “Old Nick.”