Black Dogs

Found primarily in folk tales of the British Isles, the black dog is a spectral being that appears mainly and night and is taken to be an omen of death. The black dog legend has no definite origin. It has been supposed that the legend was born out of mythological stories pertaining to Cerberus or Garm, both of whom where guardians of the underworld in Greek and Norse myth, respectively. Black dogs are always shown to be malevolent creatures, though the Barghest of Yorkshire is said to be directly harmful to those who see it. Though most are seen as portents of death, some can act benevolently, guiding lost travelers to safety, for instance. Examples of these would be the Gurt Dog of Somerset and the Black Dog of the Hanging Hills.

One of the most interesting stories involving the black dog legends is an account out of Bungay in Suffolk. The story is taken from "A Straunge and Terrible Wunder" by the Reverend Abraham Fleming in 1577 and transcribed here:

"This black dog, or the divel in such a linenesse (God hee knoweth al who worketh all,) runing all along down the body of the church with great swiftnesse, and incredible haste, among the people, in a visible fourm and shape, passed between two persons, as they were kneeling uppon their knees, and occupied in prayer as it seemed, wrung the necks of them bothe at one instant clene backward, in somuch that even at a mome[n]t where they kneeled, they stra[n]gely dyed."

Not all black dogs related to superstition are spectral in nature, though. In medieval Europe, it was the belief that the first person to die and be buried in a cemetery had to protect those that followed him. Therefore, they would kill a large, black dog and buried it first so that it could provide the protection. Magician and occult writer Heinrich Cornelius Agrippa is said to have owned a black dog that served as his familiar. The blood of a black dog was looked upon as a kind of good luck charm in parts Asia. In Japan, black dogs were once sacrificed in order to bring rain.

Sources:
Black dog (ghost); Wikipedia
The Supernatural Book of Monsters, Spirits, Demons, and Ghouls; Alex Irvine