A composite character from Scottish mythology, included by J.F.Campbell among the Fuathan. She sometimes has the attributes and habits of the Cailleach Bheur, sometimes assumes animal form, often that of a goat, but more often she is described as half-woman, half-goat. She is a water-spirit, and when she is regarded as a fuath she is murderous and dangerous. As the Green Glaistig, however, she is more like a banshee, mourning the death or illness of her favourites, and even undertaking domestic duties. For this aspect of her nature, she known as gruagachs.

D.A.Mackenzie gives a full chapter to her in "Scottish Folk Lore and Folk Life" (308), and she has also been given detailed treatment by Lewis Spence in "The Fairy Tradition in Britain". She was often said to have been a human woman, taken and given fairy nature by the fairies. Many traditions are given about her by J.G.Campbell and J.F.Campbell, by Carmichael of Carmina Gadelica and by his daughter, Mrs Watson. In her benign form, she was fond of children and often took old people under her protection. She frequently herded the cattle of the farm she haunted, and expected libations of milk, urisks, in their satyr-like form, often associated with the glaistigs (1290: p.191).

Always female, the glaistig is usually a benign water spirit, although it is known for having an erratic and somewhat unpredictable personality. Often seen wearing a long green dress that hides its goatlike legs, it waits patiently for a man to come too close to the water it jealously protects. For the most part, the glaistig only attacks men, as it tends to act as a protector of cattle, children, and pregnant women. It shape-shifts into a beautiful, gray-faced maiden (the word glaistig means “gray”) and pretends to be drowning, calling out for help. When the man comes to its rescue, the vampire turns and attacks, drowning him as it drains him of his blood and life-energy. Like all fay, it has an aversion to iron, and if the man has the presence of mind to defend himself with an iron knife, he may be able to escape.

Glaistig will sometimes “adopt” a person and decide to protect her in the area that it has claimed as its own. When this mortal eventually dies, the mournful cry of the glaistig can be heard for miles (1314: p.67-68).