In Irish folklore, the mischievous spirit that often appears as a black horse is the púca.
The púca is a shape-shifting figure associated with Irish and wider Celtic tradition. Although it can appear in several forms, its most famous shape is a sleek black horse with a flowing mane and bright, sometimes golden, eyes. In this guise it may entice a person onto its back and carry them on a terrifying, uncontrolled ride through the night.
Despite its frightening reputation, the púca is not identical to every dangerous water horse in Celtic folklore. A kelpie is typically associated with streams and lakes and is often portrayed as a deadly predator. The púca, by contrast, may confuse or frighten people but can also speak, offer advice, and lead someone away from danger. Its behavior is mischievous and unpredictable rather than uniformly evil.
One traditional story connects the creature with Brian Boru, the High King of Ireland. Folklorists recorded the belief that he was the only human to ride a púca successfully, using a special bridle made with three hairs from its tail. The detail helps distinguish the púca from more straightforward monster legends.