Barghests are a race of fiends in the Dungeons & Dragons multiverse who can basically be described as goblinoid werewolves who devour souls. Uninspiring as that might sound, they're actually one of the oldest fiendish races in D&D to have their own distinct identities. True, it's not a very exciting niche - they're overshadowed by daemons when it comes to the inhabitants of Gehenna - but it's solid enough to survive even in the shift between the Great Wheel and the World Axis. Their planetouched offspring are Worghests.
Who are they?
Barghests are a fiendish race native to Gehenna, resembling large goblins with the ability to shapeshift into fiendish-looking dogs or wolves. Though numerous and intelligent, they are not major players on the planes, if only because they tend to spend most of their time squabbling amongst themselves. Like the Nabassu breed of demon, barghests have a life cycle in which their juveniles are forcibly transported to the mortal world, where they must seek out and consume the flesh and souls of humanoids in order to slowly build up their power and attain full maturity; if they survive that long, they return to the Lower Planes and enter proper adult society.
Barghests have a number of innate Enchanter spells, require magical weapons to hit, and are immune to fire - but if you can somehow force their canine for into a sufficiently large source of fire, they are forcibly transported back to Gehenna.
Traditionally, barghests are said to rule over goblin tribes, due to their powerful magical abilities, but in 5e, it's stated that barghests actually prefer to prey on goblins. There is a sort of logic to this change; barghests are basically extraplanar goblin werewolves, and werewolves in D&D have long been characterized as being compelled to feed on human flesh above all other possible sources, so it's basically doubling down on the idea of barghests being goblin werewolves. It's still kind of stupid.
And in 4e?
The overt planar influences are removed from the World Axis barghest, who are instead soul-sucking goblinoid werewolves that are born seemingly at random to goblinoid mothers - the goblins of the World Axis view this as a blessing from Bane, and they are particularly common on Chernoggar. One of the biggest changes this causes is that barghests explicitly can resemble any of the core goblinoid breeds, with the two statted versions consisting of the Savager (whose goblinoid form is that of a bugbear) and the Battle Lord (hobgoblin).
Where did they come from?
The barghest debuted for Advanced Dungeons & Dragons 1e in Dragon Magazine #26 in 1979, with a response in the letters column in Dragon #30 confirming that it had been written by Gary Gygax himself. It would subsequently be canonized with the Monster Manual II for that edition, with Jeff Grubb mentioning their presence in Gehenna in the original Manual of the Planes.
Barghests were boosted to 2nd edition status in the Planescape boxed set, with further details in the Planes of Conflict boxed set.
This descent continued into Dungeons & Dragons 5th Edition, where barghests would not appear until Volo's Guide to Monsters.