The Nix are supernatural water beings in the folklore of Scandinavia, Germany, and Switzerland. The name “Nix” is derived from the Old High German word nihhus, meaning a crocodile, and was a vast water monster. These beings still retain the monstrous elements in their descriptions but are becoming more and more aligned to the spirit form as their folklore develops.
The females are described as being beautiful women above the waist and having the tail of a fish in the manner of a mermaid, but he Nix usually inhabits freshwater. However, it is a malicious, predatory being that uses the beautiful image to entice mortals to their doom in the waters. Their more usual image was wizened with green skin, teeth, and hair, or even as a massive, gray water-horse. There are different descriptions according to the region, and while the above is a general one, in Iceland and Sweden the description of the Nix is more like that of the centaur in the classical mythology of Greece and Rome. However, in the Slavic mythology of Eastern Europe, the female beings are described as resembling the Siren of classical mythology, with the torso and head of a woman but the legs and wings of a bird. In the same region, the male Nixes have the torso and head of a wizened old man but the body and brush of a fox and the hooves of a horse.
Rose, C. Giants, Monsters, & Dragons: An Encyclopedia of Folklore, Legend, and Myth. Norton Publishing Company. 2001.