Funny word, I know. This is a lesser known creature, I say that because if you even mention it most folks their eyes go a bit glazed over. The basic explanation is that a Barghest is a type of troll.
Barghest, the name used more commonly in the north of England, especially in Yorkshire, to a legendary monstrous black dog with huge teeth and claws. Though there are varying names given to this creature such as, Berg-geist (German) mountain demon, Bar-geist or bear demon... among other names. Along with the slight spelling changes, the creature in some cases is referred to as either a ghost or a household elf [Insert Harry Potter joke here], especially in Northumberland and Durham.
It's said the Barghest, frequents a remote gorge named Troller's Gill which is a limestone gorge close to the village of Appletreewick in Yorkshire Dales; Legend has it that the Gill is the home of trolls and the mythical monstrous black dog. Being a nocturnal spectre, the appearance is said to be a portent of death.
Now, in the mainstream of things, as mainstream as Dungeons & Dragons can get, a Barghest is portrayed as a goblin-like creature that comes from the plane of Ghehenna to feed on humans. Doesn't that sound delightful? The evil creatures can change in form to appear as a dog or wolf, or at times a large goblin at will.
The Barghest was introduced in The Dragon magazine (#26), following this it made an appearance in the first edition Advanced Dungeons & Dragons game's original Monster Manual II... it continued to make an appearance in each edition released.
In 1979, Issue #26 of The Dragon, in the "Dragon's Bestiary" feature by Gary Gygax, the billing was "the return of an old and much-requested column" formerly known as "Featured Creature", the barghest are described as lawful evil in alignment and the article continued to say "Of the various members of the deodands inhabiting the rifts of the planes of Gehenna, the barghest is certainly the most common and one of the worst." It further states that a barghest's natural shape is similar to that of a large goblin, and that it can assume the form of a large war dog or wild dog at will, sometimes giving barghests the the misnomer of "devil-dogs".
In 1983, Monster Manual II was released, the barghest within the world of Dungeons & Dragons is said to live upon the plains of Gehenna in "isolation from one another, each having it's own stronghold and force of servitors, ruling a smoking right despotically." The spawn of the barghest, a litter of six, is sent to the Prime Material Plane to feed. Living in pairs near isolated human communities or with goblin bands, always keeping their natural form amongst the goblins. The distinct difference between a barghest and a goblin are that the barghest's eyes glow orange with the creature becomes excited; in their canine form the barghest can become rather difficult to notice when motionless, and nearly "impossible to tell from a normal dog, except that other dogs will fear and hate it, attacking at every opportunity." It's said that barghests are worshiped by goblins, going to great lengths to to provide human sacrifices, as well as fearing and serving them; barghests do not take this unheeded, slaying the powerful enemies of the goblins and enriching their host's treausre.
Magical abilities in this sort of world is not uncommon, the barghest have the abilities to charm other creatures, project illusions, and affect other creature's emotions. Barghests are unable to be harmed by fire, but the canine form can be sent back to it's home plain if struck by a magical fire attack. When a barghest slays and devours a human, it becomes largers and more powerful through what is described as "the unholy vampirism attendant upon the slaughter of humanity."
Appearance wise, the skin begins to darken upon growth, from yellow towards a bluish red, at some point they become a deep blue. Once a barghest becomes fully grown, it returns home to Gehenna to find its ow reeking valley rift.
Each editions offers a few more tidbits of description from the life cycle to their attack behaviors.
For being a bit more obscure on the list of creatures, the barghest is a rather interesting and offers quite the vast amount of lore.