Supernatural creatures supposedly found all over the British Isles, especially on dark, deserted roads. They appear to be red-eyed dogs with solid black coatss; they are also quite large -- about the size of a calf. They do not bark, growl, howl, or make any sounds at all, aside from the clicking of their claws. They seem to emanate an aura of utter despair, perhaps because it is said that anyone who sees a Black Dog is destined to die soon.
Some legends say that the only defense against the powers of the Black Dogs is to have a companion with you when you travel -- supposedly, the coal-black canines won't appear to more than one person. However, other stories say that the dogs are able to manifest to only one person while remaining invisible to the other, or that the dog will appear to someone while his friend happens to be looking the other way.
A Scotsman named Ean MacEndroe of Loch Ewe is said to have once rescued one of the fae from danger; in return, he and his descendants were given immunity from the Black Dogs for all eternity. So if you have to travel on dark, deserted roads in Britain, try to get one of his modern kinfolk to go along with you...
Research from http://www.pantheon.org/articles/b/black_dog.html