The Element Encyclopedia of Witchcraft: The Complete A-Z for the Entire Magical World - Judika Illes 2005
These are female German water spirits and freshwater mermaids. Their male counterparts are the Nix. They live in societies that parallel those of humans, in underwater cities.
Male nixes have a bad reputation amongst people; they have green teeth and resemble drowned corpses. Nixies, on the other hand, are typically described as seductively beautiful although, as they are shape-shifters, their appearance may be a matter of choice. (In addition to appearing human, gray horses are favored forms.) Nixies reputedly entice mortal men to their doom.
Nixies typically manifest as mermaids, but have a passion for shopping and so like to attend local fairs and markets. No “Little Mermaid” angst regarding lack of legs for nixies. When they wish to walk on land, they sprout legs; when they wish to live in the water, legs are replaced with tails, just like that. Allegedly, the clue to the true identity of two-legged land-walking nixies is dripping water: they are always wet. Once upon a time, their apron strings were always soaked; now that aprons are out of style, it might be any item of clothing.
“Nixy” derives from the Old High German nihhusa, translated as “female water sprite.” (The male is nihhus.) Various water spirits throughout Northern and Western Europe have similar names; most are threatening and malevolent.