1 Mordekai: Draak, would you care to name a monster beginning with Y, to show Lambert how it’s done?
2 Draak: Yeth hound.
2 Lambert: Aren’t they based on legends of ghostly black dogs from various cultures?
3 Kyros: They’re the source of legends of ghostly black dogs.
4 Kyros: Legends of ghostly black dogs always have a foundation in truth.
4 Lambert: Sounds a bit Grim.

Yeth hounds are monstrous, evil black hounds in various editions of Dungeons & Dragons. They are based on the real world mythical dogs of the same name from the folklore of the Devon region of England, although the Devon yeth hounds are said to be headless dogs embodying the spirits of unbaptised dead children, including stillbirths and terminated pregnancies. They wander the woods at night, wailing horribly. To hear their cry is an omen of impending death.

(In typical D&D fashion, the game has adopted the name from real world mythology, but mutated the description into something more appropriate to fight with swords and spells.)

Yeth hounds are also know as yell hounds in some legends, and seem either identical or closely related to wisht hounds (also sometimes wish hounds), said to roam Dartmoor in southern Devon. The word yeth is a dialect spelling of "heath", indicating the dogs' moorland habitat, while wisht means "ghostly" or "haunted". The legends of these dogs or ones similar to them are likely one of the inspirations for Arthur Conan Doyle's Sherlock Holmes novel, The Hound of the Baskervilles.

Yeth hounds are a specific local example of the widespread legends of spectral black dogs. Many are attested in the folklore of different parts of England, and a few other examples from around the world.

Specifically interesting in a D&D context are the myths of a monstrous black dog from the Northumberland area, known as the Barghest. Not content with using the name of one black dog, D&D also uses barghest as the name of a monster, but in this case mutates it to be an extraplanar shapeshifting goblin-dog, rather than a ghostly hound.

Another variant on the theme of black dogs is the church grim, a collective name for several related legends spanning England and Scandinavia. The grim is a spectral black dog said to haunt churchyards, often as the result of a sacrifice of a living dog, to guard the churchyard and graves within from sacrilegious vandalism. Variously, the legends say that the spirit of the first being buried in a new churchyard must guard it, so a dog was sacrificed rather than condemn a human to such a task. Besides guarding the yard, the grim also presaged death in the community, and would either ring the church bell or appear as a vision before someone died. (This aspect of the legend was used by J.K. Rowling in Harry Potter and the Prisoner of Azkaban.)

Another interesting one is Moddey Dhoo, a black dog said to haunt the Isle of Man. It's not listed as a connection in either Wikipedia article, but... a dog related to ghostly activity, named Moddey Dhoo? Is there something for more sinister about Scooby-Doo that we should know about??