The mordy dhoo, also known as the dobhar-chú, is a legendary black "dog" in Celtic folklore. It is said to inhabit remote areas, forests and mountains, or near fresh-water lochs. The mordy dhoo is similar in appearance to the Black Shuck of East Anglian folklore, known for guarding an island monastery in England at one point, in the 16th century.
The most common description is of a spectral hound, gigantic and black with glowing red eyes. Some descriptions say that the mordy dhoo has webbed feet and the size of a horse, and others refer to it as having a long body and a long, bushy tail.
The mordy dhoo's name is thought to be derived from old Irish or Gaelic elements. "Dobhar" means "dark water," and "chú" means "hound" in Irish. The Isle of Man's Manx Gaelic translation is "moddey dhoo."
The mordy dhoo is said to be a malevolent creature, often associated with death and misfortune. It is said that the mordy dhoo will chase and attack anyone who encounters it, and that its bite is fatal. In some legends, the mordy dhoo is said to be the guardian of a hidden treasure, and that anyone who finds the treasure will be cursed. In Irish folklore, the mordy dhoo is sometimes considered an avatar of Manannán mac Lir, a powerful sea deity.
According to Celtic folklore, the mordy dhoo is most likely to appear at night, and especially during storms. It is said that anyone who hears the sound of the mordy dhoo's howl should run for their lives, as it is a sign of imminent death.
Despite its fearsome reputation, the mordy dhoo is also said to have some positive qualities. In some legends, it is said to be a protector of the innocent and a guide for lost souls. In other legends, it is said that the mordy dhoo only appears to those who are strong and brave, as a test of their courage.