Black Dogs

"Black dogs appear throughout British folklore as supernatural creatures who are met on the road by unwary travellers. The calf-sized dog is generally described as being covered with black shaggy hair and having red glowing eyes. Most accounts describe it a ferocious and menacing, although others spaek of the dog as the guardian of the place that it haunts. In the days before metalled roads, most wayfarers used lonely tracks and drover's roads which were kept open by the driving of cattle and other beasts. it is widely assumed that Roman roads were the first roads in Britain, but this is not so.Well-marked causeways were present from the Neolithic and Bronze Age when agriculture required causeways with high embankments as ways of leading animals from field to field. Black dogs often make there apperance upon these ancient roads, near to burial mounds, such as Wambarrows in Somerset. Some of the black dogs are said to guide lost travellers along the way, althoug others can turn vicious, biting and mauling if attacked. It would appear that a black dog is generally a spectral or spirit animal that demarks the boundaries as a guardian and challenges travellers at certain points upon the road.

"In explicable car crashes that happen on empty roads are often believed to have been caused by the black dog. Celtic traditions about spectral hounds and other dogs abound. The most famous literary dog in this genre is the fearsome 'Hound of the Baskervilles,' invented by Sir Arthur Conan Doyle for the Sherlock Holmes' story of that title. Interestingly, the term 'to have the black dog' is a colloquial expression for someone who is feeling depressed."

-The Element Encyclopedia of Magical Creatures