The males can assume many different shapes, including that of a human, fish, and snake. The females are beautiful women with the tail of a fish. When they are in human forms, they can be recognized by the wet hem of their clothes
The names are held to derive from Common Germanic *nikwus or *nikwis(i), derived from PIE *neigw (wash). It is related to Sanskrit nḗnēkti (wash), Greek νίζω nízō and νίπτω níptō, and Irish nigther
The form neck appears in English and Swedish (näck or nek). The Swedish form is derived from Old Swedish neker, which corresponds to Old Icelandic nykr (gen. nykrs), and nykk in New Norwegian. In Finnish, the word is näkki. In Old Danish, the form was nikke and in modern Danish and Norwegian Bokmål it is nøk(ke). The Icelandic word nykr is also used for hippopotamus.
In Middle Low German, it was called necker and in Middle Dutch nicker. The Old High German form nihhus also meant crocodile, while the Old English nicor could mean both a water monster and a hippopotamus".
The spirit has appeared in the myths and legends of all Germanic peoples in Europe.
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").
The German epic Nibelungenlied mentions the Nix in connection with the Danube, as early as 1180 to 1210.
Nixes in folklore became water sprites who try to lure people into the water. . The Nixes are portrayed as malicious in some stories but harmless and friendly in others.
By the 19th century Jacob Grimm mentions the Nixie to be among the "water-sprites" who love music, song and dancing, and says "Like the sirens, the Nixie by her song draws listening youth to herself, and then into the deep."
According to Grimm, they can appear human but have the barest hint of animal features: the nix had "a slit ear", and the Nixie "a wet skirt". Grimm thinks these could symbolize they are "higher beings" who could shapeshift to animal form.
- One famous Nixe of German folklore was Lorelei. According to the legend, she sat on the rock at the Rhine which now bears her name, and lured fishermen and boatmen to the dangers of the reefs with the sound of her voice.
- In Switzerland there is a legend (myth) of a sea-maid or Nixe that lived in lake Zug (the lake is in the Canton of Zug).
The Rhine maidens Wellgunde, Woglinde, and Floßhilde (Flosshilde) belong to a group of characters living in a part of nature free from human influence. Erda and the Norns are also considered a part of this 'hidden' world.
They are first seen in the first work of the Nibelungen cycle, Das Rheingold, as guardians of the Rheingold, a treasure of gold hidden in the Rhein river. The dwarf Alberich, a Nibelung, is eager to win their favour, but they somewhat cruelly dismiss his flattery. They tell him that only one who is unable to love can win the Rheingold. Thus, Alberich curses love and steals the Rheingold. From the stolen gold he forges a ring of power.
Further on in the cycle, the Rhine maidens are seen trying to regain the ring and transform it back into the harmless Rheingold. But no one, not even the supreme god Wotan, who uses the ring to pay the giants Fasolt and Fafner for building Valhalla, nor the hero Siegfried, when the maidens appear to him in the third act of Götterdämmerung, will return the ring to them. Eventually Brünnhilde returns it to them at the end of the cycle, when the fires of her funeral pyre cleanse the ring of its curse.
- The Yellow Fairy Book by Andrew Lang includes a story called The Nixie of the Mill-Pond in which a malevolent spirit that lives in a mill pond strikes a deal with the miller that she will restore his wealth in exchange for his son. This story is taken from the Tales of Grimm.
- The legend of Heer Halewijn, a dangerous lord who lures women to their deaths with a magic song, may have originated with the Nix.
- In a fictional depiction, the Rhine maidens are among the protagonists in the four-part Opera Der Ring des Nibelungen by the composer Richard Wagner, based loosely on the nix of the Nibelungenlied.
- Grimm, Jacob (1835). Deutsche Mythologie (German Mythology); From English released version Grimm's Teutonic Mythology (1888); Available online by Northvegr © 2004-2007: Chapter 17, page 11; Chapter 33, page 2. File retrieved 4 June 2007.
- Hellström, AnneMarie. (1985). Jag vill så gärna berätta.... ISBN 91-7908-002-2
- Karlsson, S. (1970). I Tiveden, Reflex, Mariestad.