BLACK DOG noun : informal noun: black dog; plural noun: black dogs

1. used as a metaphor for melancholy or depression. "I'm very happy, but the black dog is there, lurking around the corner"

Origin: late 18th century: figuratively from a cant name used during Queen Anne's reign (1702–14) for a base silver coin (usually a bad shilling).

A black dog is a spectral or demonic entity found primarily in the folklore of the British Isles. The black dog is essentially a nocturnal apparition, some of them shapeshifters, and are often said to be associated with the Devil or described as a ghost or hellhound. Its appearance was regarded as a portent of death. It is generally supposed to be larger than a normal dog and often has large glowing eyes. It is sometimes associated with electrical storms (such as Black Shuck's appearance at Bungay, Suffolk) and also with crossroads, places of execution and ancient pathways.

The origins of the black dog are difficult to discern. It is uncertain whether the creature originated in the Celtic or Germanic elements of British culture. Throughout European mythology, dogs have been associated with death. Examples of this are the Cŵn Annwn (Welsh), Garmr (Norse) and Cerberus (Greek), 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 generally regarded as sinister or malevolent, and a few (such as the Barghest and Shuck) are said to be directly harmful. They may also serve as Familiar Spirits for witches and warlocks. Some black dogs, however, such as the Gurt Dog in Somerset and the Black Dog of the Hanging Hills in Connecticut are said to behave benevolently. Some, known as guardian black dogs, guide travelers at night onto the right path or guard them from danger.

They're also cuddly as hell.

Sometimes...