Black Dogs go by lots of different names, like Shuck oop north and Girt Dog around south west. Probably the most famous one is the Moddy Dhoo at Peel Castle on the Isle of Man.
They always struck me as a bit unpredictable, I mean, if you see one, you'll die, but then they can also protect you. Bit of a risk involved there, methinks.
'The many phantom dogs of local legend are almost invariably large black shaggy ones with glowing eyes; those which appear only in this form are simply called 'the Black Dog' whereas those that change shape often have some regional name such as bargest, padfoot, or Shuck. A few are said to be ghosts, but the majority are either supernatural creatures in their own right or manifestations of the Devil. They are solitary, unlike the pack of hounds forming the Wild Hunt (though these too are black) [note - the Wild Hunt dogs can also be white with red ears - H]; they usually patrol specified lanes, but some are associated with churchyards, streams, pools, gallows sites and barrows. In some districts (eg. Lincolnshire) it is said that they are harmless, or even friendly, if they are not disturbed, though in others it is an omen of death to meet one. Occasionally they guard treasure as at Dobb park Lodge (Lancashire). Another haunted a farm near Lyme Regis (Dorset) to the annoyance of the farmer, who chased it with a poker and accidentally struck the attic wall, dislodging a hidden box of coins. [note - I think it's somewhere around Lyme Regis where there's a pub wall with burnt in scratch marks down the door, supposedly from a Black Dog - H]
The idea that the Devil may appear as a Black Dog is found in several accounts of witch trials and in other printed sources. A violent storm one Sunday in August 1577 damaged the villages of Blythburgh and Burngay in Suffolk, and a contemporary tract claimed that a black dog of 'horrible shape' accompanied by 'fearful flashes of fire' was seen rushing through both churches, killing or injuring several people; it was 'the divil in such likeness' (Briggs) Another pamphlet of 1638 described the Black Dog of Newgate Gaol which would ride in the cart beside criminals going to the gallows; this was explained as the ghost of a medieval wizard, killed and eaten by starving prisoners.
Black Dog legends are common in East Anglia, the northern counties and the south west, and occur sporadically elsewhere; there is an extensive list, including modern eyewitness accounts, in Janet and Colin Bord Alien Animals, 1981......'
[A Dictionary of English Folklore by Jacqueline Simpson and Steve Roud - only purchased a couple of days ago courtesy of Amazon, and it's proving to be great fun to dip into and read snippets of]
Few other references, but that about covers it, I suppose, unless anyone wants more, or Moddy Dhoo story etc.