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.
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 Nixes, or Water-people, inhabit lakes and rivers. The man is like any other man, only he has green teeth. He also wears a green hat. The female Nixes appear like beautiful maidens. On fine sunny days they may be seen sitting on the banks, or on the branches of the trees, combing their long golden locks. When any person is shortly to be drowned, the Nixes may be previously seen dancing on the surface of the water. They inhabit a magnificent region below the water, whither they sometimes convey mortals.
A girl from a village near Leipzig was one time at service in the house of a Nix. She said that everything there was very good; all she had to complain of was that she was obliged to eat her food without salt. The female Nixes frequently go to the market to buy meat: they are always dressed with extreme neatness, only a corner of their apron or some other part of their clothes is wet. The man has also occasionally gone to market. They are fond of carrying off women whom they make wives of, and often fetch an earthly midwife to assist at their labour.
THE FAIRY MYTHOLOGY BY THOMAS KEIGHTLEY (1850)