"The long-haired one," from the Gaelic gruag, a wig. The gruagach was a fairy being with protective duties in Scottish legends, apparently of either sex, but generally female. The gruagach was particularly associated with cattle, and milk was laid aside for him or her every evening—otherwise no milk would be given at the next milking. Usually this being was of a beneficent nature, although it occasionally made mischief by loosing the cattle so that the herders had to get up, sometimes several times during a night, to tie them up. This apparently caused the gruagach much impish delight.
Among the many stories of fairies, there are tales in different parts of Scotland about the gruagach. It seems that this fairy commonly had long hair and was well dressed, whichever sex it happened to be.
Thompson, Francis. The Supernatural Highland. London: Robert Hale, 1976.
"Gruagach." Encyclopedia of Occultism and Parapsychology. . Encyclopedia.com. (December 16, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/gruagach
"Gruagach." Encyclopedia of Occultism and Parapsychology. . Retrieved December 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/gruagach