The Glaistig is one of the most mixed-up spirits in Scottish folklore — mixed in body and mixed in temper. From the waist up she is a beautiful woman, pale, with long golden or dark hair. From the waist down she is a goat, and she wears a long flowing gown of green or grey-green to hide the fact. The hooves, somehow, always show.
In the western Highlands and islands, she was known above all as a guardian of cattle. A Glaistig attached to a farm would watch the herd through the night, keep calves from straying onto the cliffs, and drive the cows home from the hill. Some tellings say she would even tidy the house or finish the chores, like a brownie. In return, she wanted one thing: milk. A portion of the evening milking was poured into a hollowed stone — a 'glaistig stone' — or left in a bowl at the byre door. Forget it, and the herd would suffer. The cows would go dry, a calf would fall, the luck of the farm would sour.
Many Glaistigs were said to have once been human — noblewomen, or the ladies of great houses — who had been put under a fairy spell and given a goat's nature. That may be why they were often tied to a particular castle or family, haunting its fields for generations, fond of some descendants and spiteful to others.
Because the other side of the Glaistig was dangerous. If she was insulted, cheated or harmed, she was quick to take revenge. Some tellings make her a seductress who lured men to dance with her and then drank their blood — which has led to her being confused, often, with the Baobhan Sìth. But they are not the same being. The Baobhan Sìth is a creature of the dance and the hunt, a green-gowned killer of travellers. The Glaistig is a creature of the farm, bound to a place and a herd, capable of great loyalty as well as great cruelty. Cousins, perhaps. Not twins.
One old story tells of a man who caught a Glaistig and would not let her go until she promised to build him a house. She built it — but she also cursed his family so that none of them would ever prosper. Another tells of a Glaistig who wailed for days when the family she guarded fell on hard times. Both are true to her: a spirit who cared, fiercely and dangerously, about the people and beasts she chose.
She has kin elsewhere in Scottish lore too — the Ghillie Dhu of the birch woods, another half-wild guardian of a particular patch of land, kind to children and hard on anyone who wronged his trees.
Wear the Glaistig and you wear the green maiden at the byre door: a guardian worth every drop of milk, and a very bad enemy to make.