kelpie (n.)

1747, Scottish, of unknown origin, perhaps related to Gaelic colpach "heifer, steer, colt;" colpa "cow, horse." The Lowland name of a demon in the shape of a horse that was reputed to haunt lakes and rivers and to delight in causing drownings. But unlike its equivalents in Danish (nøkken) and Icelandic (nykur), it occasionally was benevolent, especially to millers by keeping their streams running.