From Collin de Plancy's Dictionnaire Infernal

The neck, nickar, nicor, or nixie or nokken (German: Nixe; Norwegian: nøkk; Swedish: näck) is a demon or spirit in the Dictionnaire Infernal. In Scandinavian, Teutonic and Saxon mythology, Nickar are malevolent water spirits who drown people, torment fisherman, tip over boats and throw them to the tops of shore-side trees. According to mythology, Nickars or Hnickars patrol the waters of Scandinavian fjords, where they cause tempests, hurricanes, hailstorms and blizzards. They have fish-like tails and often sit by the water combing each other's long green or blonde hair with seaweed, playing a harp or singing. Like the Sirens, they drag to the bottom of the sea anyone who ventures too close.

Nickar are descendended from the Mermen and Mermaids, or from the Nymphs of the Elbe and the Gaal, the Nixies (Teutonic) which are kind, loving benevolent water spirits, known to rescue wayward fishermen. These lesser water divinities will sometimes leave the water to attend nearby celebrations and dance or fire festivals, where they dance and greet local fishermen.

It has been suggested that "Old Nick", one of the colloquial English names for the devil, is derived from Nickar, but it seems more likely to be a contraction of the personal name "Nicholas" instead.

Names and etymology

The names are held to derive from Common Germanic *nikwus or *nikwis(i), derived from PIE *neigw ("to wash"). It is 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 nøkk. The Icelandic nykur is a horselike creature. In Middle Low German, it was called necker and in Middle Dutch nicker (c.f. 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" and a "hippopotamus". Common bynames are the Swedish Strömkarlen and the Norwegian Fossegrim. Since the Scandinavian version can transform himself into a horse-like kelpie, he is also called Bäckahästen (the "brook horse").