The hellhounds in my own writing are based on a rich, Europe-wide mythological tradition of underworldly, or otherworldly, canines.
Most people know Cerberus (Kerberos), the multi-headed rascal briefly subdued by Hercules (Heracles) in the ancient Roman and Greek traditions.
Germanic mythology has a richer selection of dogly guardians and hunters - many of them, like Cerberus, portrayed as greedy and vicious brutes.
Black dog apparitions are found mostly in Britain. The apparition of a lone black dog can be a positive, negative or neutral experience. Solitary travellers have reported a dog appearing, walking beside them for a while and then vanishing.
Many places in Britain are said to be haunted - or more likely guarded - by a ghostly dog. A friend of mine tells me his family has its own black dog, who turns up to forebode some disaster or another.
A black dog famously attacked the church in the town of Bungay, Suffolk, during a thunderstorm in 1577. Another dog (or the same one) simultaneously attacked nearby Blythburgh, leaving charred scorchmarks on the church door. (It could have been a lightning strike, but that's not as much fun, is it?)
Black dogs have turned up as guardians of churches and graveyards, too. A 'church grim' was a sacrificial dog who patrolled a churchyard, according to a superstition that the first creature interred there was doomed to guard the land. In Scandinavia lambs were used instead. [Trubshaw 1]
Across Europe, spectral dogs often form part of an entire ghostly hunt, led by the likes of Odin, Cernunnos or Arawn in Wales - where the pack was referred to as Cwn Annwn ('the hounds of Annwn', the Welsh otherworld) and the dogs were described as white with red ears.
Humans are generally advised to avoid getting in their way for fear of being swept up or trampled.
Though some locations give them friendly-sounding nicknames like 'Gabriel Hounds' or 'Wish Hounds', there seem to be few or no examples of unequivocally benign supernatural hunting dogs.
The heavenly packs in my own writing ominously fail to buck this trend...
We have a word for something which is half a wolf and half a human, and that's dog. --Terry Pratchett 
A few notes on wolves to come.
Elsewhere in fiction, of course, there is the Hound of the Baskervilles investigated by Mr. Sherlock Holmes. The rock group The Darkness immortalised the Blythburgh black dog in their song Black Shuck.
 Trubshaw, Guardians of the corpse ways; see further reading.
 Really a corruption of 'Wisht Hounds': fiery-eyed black dogs seen around Dartmoor, also known as 'Yeth Hounds', and not very friendly at all.
 Terry Pratchett quoted in this interview, though he's said it in numerous ways in various places.