kelpie \KEL-pee\,
noun:
(in Scottish legends) a water spirit, usually having the form of a horse, reputed to cause drownings or to warn those in danger of drowning.
"Well, in this pot there lived a kelpie." "What's a kelpie, Kirsty?" again interposed Allister, who in general asked all the necessary questions and at least as many unnecessary. "A kelpie is an awful creature that eats people."
-- George MacDonald, "Ranald Bannerman's Boyhood," 1871
I could well understand the story of the Water Kelpie, that demon of the streams, who is fabled to keep wailing and roaring at the ford until the coming of the doomed traveller.
-- Robert Louis Stevenson, "Kidnapped," 1886
Kelpie is of unknown origin, though it is thought to be related to the Gaelic colpach meaning "steer" or "colt." It first entered English in Scotland in the mid-1700s.
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