huldre
A type of supernatural being in the shape of a beautiful woman with a cow's tail.
All these things, it was said, must have a cause, and from ignorance of nature, joined to superstition and a lively imagination, the idea suggested itself of conjuring up beings, to whom all these phenomena might be ascribed, and who, according to the places of sojourn assigned them, were called Forest-trolls, Huldres, Mountain-trolls, Vættir, Elves, Dwarfs, Nisser, Mares, etc.
Such beings collectively.
In Norway, the Huldre drive cattle before them, which are as blue as they are themselves.
Where it comes from
Learned borrowing from Norwegian hulder, with the same meaning.
Adapted from the Wiktionary entry huldre (CC BY-SA 4.0).