From Notes and Queries, Vol. 1, No. 29, May 18, 1850, p. 468:
Shuck the Dog-Fiend -- This phantom I have heard many persons in East Norfolk, and even Cambridgeshire, describe as having seen as a black shaggy dog, with fiery eyes, and of immense size, and who visits churchyards at midnight. One witness nearly fainted away at seeing it, and on bringing his neighbours to see the place where he saw it, he found a large spot as if gunpowder had been exploded there. A lane in the parish of Overstrand is called, after him, Shuck's Lane. The name appears to be a corruption of "shag", as shucky is the Norfolk dialect for "shaggy". Is not this a vestige of the German "Dog-fiend?"
E.S.T.
"E.S.T." received an answer from one "T.T.W." of Burnley, in the issue of June 22, 1850:
"Many hundreds of persons there are in these districts who place implicit credence in the reality of the appearance of a death sign, locally termed trash or skriker. It has the appearance of a large black dog, with long shaggy hair, and, as the natives express it, "eyes as big as saucers." The first name is given to it from the peculiar noise made by its feet when passing along, resembling that of a heavy shoe in a miry road. The second appellation is in allusion to the sound of its voice when heard by those parties who are unable to see the appearance itself."
T.T.W. adds that if anyone is brave enough to face down the entity, "it usually makes its retreat with its eyes fronting the pursuer, and either sinks into the earth with a strange noise, or is lost upon the slightest momentary inattention." It is not confined to chuchyards, and ordinary weapons do no harm to it.
Robert W. Chambers thick two-volume Book of Days (1888) carries articles based on each day of the year. His article "Spectre-Dogs" is under the heading for October 11 for no apparent reason (oddly, that's the day I started typing this). He tells of the old Black Dog in Hertfordshire:
Within the parish of Tring [Hertfordshire], but about three miles from the town, a poor old woman was, in 1751, drowned for suspected witchcraft. A chimney-sweep, who was the principal perpetrator of this atrocious deed, was hanged and gibbeted near the place where the murder was effected. While the gibbet stood, and long after it disappeared, the spot was haunted by a black dog. The writer was told by the village schoolmaster, who had been "abroad", that he himself had seen this diabolical dog. "I was returning home late at night in a gig with the person who was driving. When we came near the spot where a portion of the gibbet had lately stood, we saw on the bank of the roadside a flame of fire as large as a man's hat. 'What's that?' I exclaimed. 'Hush!' said my companion, and suddenly pulling in his horse, came to a dead stop. I then saw an immense black dog just in front of our horse, the strangest looking creature I ever beheld. He was as big as a Newfoundland, but very gaunt, shaggy, with long ears and tail, eyes like balls of fire, and large, long teeth, for he opened his mouth and seemed to grin at us. In a few minutes the dog disappeared, seeming to vanish like a shadow, or to sink into the earth, and we drove on over the spot where he had lain."
Dorsetshire is also the home of Black Dogs, as Chambers reports:
So late as the year 1856, a respectable intelligent woman told the writer that she herself had seen the dog-ghost. 'As I was returning to Lyme,' said she, 'one night with my husband down Dog Lane, as we reached about the middle of it, I saw an animal about the size of a dog meeting us. "What's that?" I said to my husband. "What?" said he, "I see nothing." I was so frightened I could say no more then, for the animal was within two or three yards of us, and had become as large as a young calf, but had the appearance of a black shaggy dog with fiery eyes, just like the description I had heard of the "black dog." He passed close by me, and made the air cold and dank as he passed along. Though I was afraid to speak, I could not help turning round to look after him, and I saw him growing bigger and bigger as he went along, till he was as high as the trees by the roadside, and then seeming to swell into a large cloud, he vanished in the air. As soon as I could speak, I asked my husband to look at his watch, and it was then five minutes past twelve. My husband said he saw nothing but a vapour or fog coming up from the sea.' A case of this kind shews how even a sensible person may become the victim of self-delusion; for in all practical matters this woman was remarkably sober-minded, intelligent, and judicious; and well educated for a person of her calling -- that of sick-nurse, the duties of which she discharged in the writer's house for several weeks to his fullest satisfaction, shewing no symptoms of nervousness or timidity.
Chambers' comments demonstrate how difficult it was in the 19th century for someone (especially a woman) to report seeing something "unusual" and be taken seriously. Maybe things are not so much different today.
The Black Dog is no relic of earlier times, however. Take Ethel H. Rudkin's article, "The Black Dog," from Folklore, v. 49, 1938:
"The Black Dog walks in Lincolnshire still," writes Mrs. Rudkin. (Continue)
Theo Brown wrote a sequel of sorts to Rudkin's article, also entitled "The Black Dog," in the same journal 20 years later. Her article is more an analysis of the old stories, and she makes some interesting observations. "The most striking characteristic of the Barguest type is that it goes out of its way to show the beholder it is no normal dog." To this I might add that most ghosts ignore their human observers and pursue their same old occupations, but Black Dogs almost always give the witness a glare at least, as if to make sure the witness knows it has seen him or her.
Brown mentions that "Mrs. Rudkin herself saw the Black Dog in 1926 at the ruined Dunwich Abbey"!
Some people associate the Black Dog with graveyards because many burial grounds were founded by killing a sacrificial animal and burying it before any human bodies were interred. Hares and cats have been used, but often the sacrifice was a dog, completely black, not a single white hair on its body. The animal's ghost -- a "Church Grim" -- thereafter guarded the graveyard from desecration.
Theo Brown questions the historical validity of Church Grims but has no alternate explanation for what the Black Dogs are, ending with: "It begins to look as though our creatures are faintly echoing some half-forgotten mythology of vast antiquity."