The Kelpie is the supernatural shape-shifting water horse that haunts the rivers and streams of Ireland and Scotland. It is probably one of the best known of Celtic water spirits and is often mistakenly thought to haunt lochs. In Scotland’s Loch Ness is said to have a Kelpie, as well as Loch Neagh in Ireland.
The creature could take many forms and had an insatiable appetite for humans; its most common guise was that of a beautiful tame horse standing by the riverside - a tempting ride for a weary traveller. Anybody foolish enough to mount the horse - perhaps a stranger unaware of the local traditions - would find themselves in dire peril, as the horse would rear and charge headlong into the deepest part of the water, submerging with a noise like thunder to the travellers watery grave. The Kelpie was also said to warn of impending storms by wailing and howling, which would carry on through the tempest. This association with thunder - the sound its tail makes as it submerges under water - and storms, may be related to ancient worship of river and weather deities by the ancient Celts.