The neck, nicor, nokk, nix, nixie, nixy, or nokken (German: Nixe; Dutch: nikker, nekker; Danish: nøkke; Norwegian Bokmål: nøkk; Nynorsk: nykk; Swedish: näck; Faroese: nykur; Finnish: näkki; Estonian: näkk; Old English: nicor) are shapeshifting water spirits in Germanic mythology and folklore who usually appeared in the form of other creatures.
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Under a variety of names, they were common to the stories of all Germanic peoples, although they are perhaps best known from Scandinavian folklore. The related English knucker was generally depicted as a wyrm or dragon, although more recent versions depict the spirits in other forms. Their sex, bynames, and various transformations vary geographically. The German Nix and his Scandinavian counterparts were male. The German Nixe was a female river mermaid.
The names are held to derive from Common Germanic *nikwus or *nikwis(i), derived from PIE *neigw ("to wash"). They are related to Sanskrit nḗnēkti, Greek νίζω nízō and νίπτω níptō, and Irish nigh (all meaning to wash or be washed). The form neck appears in English and Swedish (näck or nek, meaning "nude"). The Swedish form is derived from Old Swedish neker, which corresponds to Old Icelandic nykr (gen. nykrs), and nykk in Norwegian Nynorsk. In Finnish, the word is näkki. In Old Danish, the form was nikke and in modern Danish and Norwegian Bokmål it is nøkke/nøkk. The Icelandic and Faroese nykur are horselike creatures. In Middle Low German, it was called necker and in Middle Dutch nicker (compare also Nickel or Nikkel plus Kobolt) . The Old High German form nihhus also meant "crocodile", while the Old English nicor could mean both a "water monster" like those encountered by Beowulf, and a "hippopotamus".
The Norwegian Fossegrim and Swedish Strömkarlen are related figures sometimes seen as by-names for the same creature. The southern Scandinavian version can transform himself into a horse-like kelpie, and is called a Bäckahästen (the "brook horse"), whilst the Welsh version is called the Ceffyl Dŵr (the "water horse").
The Scandinavian näcken, näkki, nøkk were male water spirits who played enchanted songs on the violin, luring women and children to drown in lakes or streams. However, not all of these spirits were necessarily malevolent; many stories indicate at the very least that nøkker were entirely harmless to their audience and attracted not only women and children, but men as well with their sweet songs. Stories also exist wherein the Fossegrim agreed to live with a human who had fallen in love with him, but many of these stories ended with the nøkk returning to his home, usually a nearby waterfall or brook. (Compare the legend of Llyn y Fan Fach in Wales.) The nøkker were said to grow despondent unless they had free, regular contact with a water source.
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The Norwegian Fossegrim or Grim, Swedish strömkarl, is a related figure who, if properly approached, will teach a musician to play so adeptly "that the trees dance and waterfalls stop at his music".
.
Under a variety of names, they were common to the stories of all Germanic peoples, although they are perhaps best known from Scandinavian folklore. The related English knucker was generally depicted as a wyrm or dragon, although more recent versions depict the spirits in other forms. Their sex, bynames, and various transformations vary geographically. The German Nix and his Scandinavian counterparts were male. The German Nixe was a female river mermaid.
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The Norwegian Fossegrim or Grim, Swedish strömkarl, is a related figure who, if properly approached, will teach a musician to play so adeptly "that the trees dance and waterfalls stop at his music".
And there's heaps more to read: https://en.wikipedia.org/wiki/Neck_(water_spirit)
-Geralt of Rivia