Dogs are associated with the underworld in several branches of folklore.
Cerberus -- Garm -- Hell Hound
According to European folklore, souls of sinners ares sometimes chased by hounds from Hell. One example of this is the legend of Jan Tregeagle, a corrupt magistrate from Cornwall. Tregeagle was born in the 1660's, and was notorious for abusing his wife and child and allegedly selling his soul to the Devil.
IMAGES: Image of Bungay's Black Dog from a 1577 pamphlet
In Greek mythology, Cerberus is the dog who guards the entrance to Hades. He is now ususally thought of as having three heads, although early descriptions of the creature claim that he has up to a hundred. He is also sometimes depicted with a snake's tail.
IMAGE: An ancient Greek vase depicting Cerberus
Garm is a hound from Norse mythology. He guards the entrance to Hel's home, the underworld, and is often said to have multiple heads, making him smilar to the Greek Cerberus. At Ragnarok, the Norse apocalypse, Garm and the god Tyr shall kill eachother.HELL HOUND

After he died, there was a great dispute over who should claim his estate s it was found that he frequently failed to pay rents and forged documents. One estate claim was taken to court, and the last witness to be called in was none other than Jan Tregeagle's ghost.
The ghost confessed to his crimes, and the story takes an even stranger turn. As the defendant, who summoned the ghost, was unable to send it back, the judge handed Tregeagle over to the Clergy.
In his life Tregeagle had given large sums of money to the Church in the hopes that they would pray for his soul's salvation, and so they decided that they would fulfill their half of the bargain, but also punish him for his sins.
They gave him the task of draining a lake - then thought to be bottomless - with a limpet shell that was full of holes. If he stopped, the Clergymen warned, the Devil would send headless hounds from Hell to chase him. One night, when a storm was raging, Tregeagle could no longer stand his task and decided to run, persued by the hell hounds. Eventually he reached a chapel, where he prayed to St. Petroc for help. The saint took him to Padstow Beach, and the Church gave him another task - weaving ropes out of sand.
The ghost took up this task, but complained so loudly that he became an annoyance to the locals. St. Petroc responded by tying him up in one of the ropes he made and carrying him to Berepper, where he was ordered to carry sacks of sand to Porthleven (he was said to have dropped on of the sacks, causing the sand to spill out and form the Loe Bar). Once again the locals complained, and Tregeagle was forced to move on.
According to legend, he was last seen at Porthcurno Cove, sweeping sand into Mill Bay.
For a similar story involving hell hounds, read the vampire entry.
Also related are the British "black dog" legends. These stories tell of demons in the forms of black dogs. A famous black dog appeared in St. Mary's Church in Bungay, Suffolk, on the 4th of August 1577. That night a storm was raging, and after a particularly loud thunderclap a black dog burst through the doors. It ran between two parishioners, killing them both. According to a pamphlet describing the incident, a man who touched the dog survived but became "as shrunken as a piece of leather left in a hot fire". The nearby Blythburgh Church was also visited by a black dog that night. Three more worshippers were killed and claw marks were left on a door. They remain there to this day.CERBERUS

The twelfth task given to Heracles by the king Eurystheus was capture Cerberus and take him, still alive, to Eurystheus' court. Hades gave Heracles permission to fulfill this task so long as he used no weapons on Cerberus - but he was able to defeat Cerberus with his bare hands, and so completed his twelfth and final task.
GARM
