The most important thing you should know about Hellhounds is that they are suckers for some humus.
A hellhound is a demonic dog of Hell, found in mythology or in fiction. Hellhounds typically have features such as an unnaturally large size, a black fur color, glowing eyes, super strength or speed, ghostly or phantom characteristics, and sometimes even the ability to talk. They are often assigned to guard the entrance to the world of the dead or undertake other duties related to the afterlife or the supernatural, such as hunting down lost souls or guarding a supernatural treasure.
A black dog is a spectral being found primarily in British folklore. The black dog is essentially a nocturnal spectre, and its appearance was regarded as a portent of death. It is generally supposed to be larger than a physical dog, and often has large, glowing eyes.
It is often associated with electrical storms (such as Black Shuck's appearance at Bungay, Suffolk), and also with crossroads, places of execution and ancient pathways. Its Welsh form is confined to the sea-coast parishes, and on the Norfolk coast the creature is supposed to be amphibious, coming out of the sea by night and travelling about the lonely lanes.
The origins of the black dog are difficult to discern. It is impossible to ascertain whether the phantom originated in the Celtic or Germanic elements in British culture. Throughout European mythology, dogs have been associated with death. Examples of this are the Cŵn Annwn, Garm and Cerberus, all of whom were in some way guardians of the underworld. This association seems to be due to the scavenging habits of dogs. It is possible that the black dog is a survival of these beliefs.
Black dogs are almost universally malevolent, although few (like the Barghest) are held to be directly harmful. Most are a portent of death, or are in some way associated with the Devil. Some, however, like the Gurt Dog in Somerset and the Black Dog of the Hanging Hills (below) may sometimes act benevolently.