The guytrash, or gytrash in British Folklore is a fell spirit which appears to someone who is going to die, much like a Banshee. It can take many forms, it is most usualy a black dog dragging a chain, but it could be a calf, a headless man, a horse, a goblin or even a Rolling Stone, (I know, Jagger frightens us all.) In Charlotte Bronte's Jane Eyre, the heroine mistakes Mister Rotchester's dog for a gytrash, "A lion like creature with long hair and a huge head."
Some families have their own hereditary guytrash, which dog (No pun intended) members of the family wherever they go. Sometimes it is not even necessary to see the Guytrash, just to hear it howl is enough to spell your doom. In some cases people have died of fright at hearing it.
The best way to avoid the guytrash is to travel in company. They only appear to solitary travellers at night. If you do see one, do not worry, they never physically harm a person. If you encounter a guytrash your fate has already been sealed.
It is not to be confused with the barguest, they are entirely separate and individual entities.
Reference: The Oxford Dictionary of English Folklore.