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A kelpie is a malevolent water spirit of the British isles, especially Scotland. They are said to take the form of young horses. They graze near lakes and rivers trying to lure unsuspecting travelers onto their backs. Anyone foolish enough to try to capture and ride the horse will be pulled down into the water and drown.
They can also take human form.
Though some tales say that they are helpful to millers by keeping the water wheel spinning.
See also: Nixie, Hippocampus