A water horse (or "waterhorse" in some folklore) is a mythical creature, such as the Ceffyl Dŵr, Capaill Uisce, the bäckahäst, each-uisge, and kelpie.

The Breton King Gradlon's magical "horse of the sea" Morvarc'h (whose name literally means "sea horse" in Breton) was said to have the ability to gallop upon the waves of the sea, in a similar fashion to the water horses of Cornish legend.

Loch Morar is reputedly home to "Morag", a lake monster that has been portrayed as a water horse.

Most Highland lochs have some kind of water-horse tradition, although a study of 19th-century literature of the time showed that only about sixty lochs and lochans merited a mention out of the thousands of bodies of water in Scotland.

The water horse that was reputed to inhabit Loch Ness gained the most mentions in Highland literature.