Canines and their relatives have long had their place in the folklore of the world; from the classic werewolf to the tanuki of Japanese myth and Celtic mythology is no different. In this case, mythological beasts tend to run in the circles of the faerie or the far beyond as ghosts.
Faerie dogs haunt the crossroads where the barriers between our mortal world and the realms of the faerie are the thinnest and most fragile. They’re said to be bright green and guard their doors quite fiercely. They say that the faerie dog will bark only twice in warning to trespassers; if you hear the their third bark then you’re doomed to die.
The Church Grim was made famous in Harry Potter and the Prisoner of Azkaban as an omen of death in the form of a giant black dog. The original myths of the Grim paint the creatures as a little more friendly; they’re guardian spirits tied to a particular church or graveyard who oversee the welfare of their lands and rather enjoy loudly ringing the church bells. The black dog is the most common form for them but they can take shape of several animals including a ram, horse, or raven.
Perhaps most monstrous of all the Celtic dogs is the Barghest, the legendary spectre that is said to haunt pathways and roads and preys upon those who travel alone. It can also take many forms, from a giant dog with fiery eyes and massive claws to a headless sorcerer who vanishes in a cloak of flames. Like the bean sidhe the Barghest can foretell the death of an important individual and like the traditional vampire cannot cross running water.