The water horse (or each uisge in Gaelic) was a mythical creature in Celtic folklore that lived near a lake or the sea. A victim who tried to ride on its back would be carried into the water and drowned. It could also change its shape.

In Orkney the creature was called the Nuggle, and in Shetland a similar creature was called the Shoopiltee.

See also the Kelpie, a similar mythical beast that inhabited rivers and streams.