A shape-shifting water spirit inhabiting the lochs and pools of Scotland. It has usually been described as appearing as a horse, but have the ability to transform themselves into non-equine forms. They can take on the outward appearance of human figures, but may betray themselves by the presence of water weeds in their hair.
One of the few stories describing the creature in female form is set at Conon House in Ross and Cromarty. It tells of a "tall woman dressed in green", with a "withered, meagre countenance, ever distorted by a malignant scowl", who overpowered and drowned a man and a boy after she jumped out of a stream. Other accounts describe the kelpie when appearing in human form as a "rough, shaggy man who leaps behind a solitary rider, gripping and crushing him", or as tearing apart and devouring humans.
A folk tale from Barra tells of a lonely kelpie that transforms itself into a handsome young man to woo a pretty young girl it was determined to take for its wife. The girl recognises the young man as a kelpie and removes his silver necklace (his bridle) while he sleeps. The kelpie immediately reverts to its equine form, and the girl takes it home to her father's farm, where it is put to work for a year. At the end of that time the girl rides the kelpie to consult a wise man, who tells her to return the silver necklace. Once again transformed into the handsome young man she had first met the wise man asks the kelpie whether if given the choice it would choose to be a kelpie or a mortal. The kelpie in turn asks the girl whether, if he were a man, she would agree to be his wife. She confirms that she would, after which the kelpie chooses to become a mortal man, and the pair are married.