Glabrezu are a species of demon (Tanar'ri) native to the multiverse of Dungeons & Dragons, characterized by their unique combination of brains and brawn. To look at them, one sees a typical demonic bruiser; a glabrezu resembles a giant humanoid dog, sometimes with horns, and always with four arms; the uppermost pair end in crushing, rending, crab-like pincers, whilst the lower pair are smaller, more delicate, and resemble human hands.
Unusually for demons, glabrezu are seductive and guileful; they will certainly rend and destroy like any demon when the opportunity presents itself, but they favor the subtlety of corrupting mortals over simple ravening, and thusly they like to use guile, trickery and manipulation to bring doom to mortals - in this field, they are considered second in skill and prowess only to the succubus. Unlike their rivals, the glabrezu does not offer pleasures of the flesh but offers power, wealth, and respect undreamed of by even the most ambitious person - though, what's that saying about power being better than sex and wealth being quite the aphrodisiac?
Their success rate is unusually high, considering that glabrezus lack the inherent means to disguise themselves as anything other than the monstrous dog-fiends they are. It most likely stems from the fact that, by demon standards, glabrezus are actually very reliable and willing to uphold their word, which makes them a prime candidate for conjurers willing to delve the Lower Planes. Of course, you always get those morons who presume "smart demon" means "still way dumber than a devil" and thus think themselves mentally superior to the glabrezu - which is a recipe for disaster, since the glabrezu is up there with the marilith in the ranks of "demons who are scary-smart". That said, they don't entirely depend on ambitious summoners, and are quite willing to find their own mortals to corrupt, presenting their "gift" to this person whether or not they want it. In some cases, glabrezus never reveal themselves to a person, relying on their telepathy to speak directly in the victim's mind; they may be direct about their nature, or make the person believe he is going insane.
For unknown reasons, glabrezu get on particularly well with the Cult of Lolth, perhaps because she and they are amongst those rare denizens of the Abyss who favor subtlety and guile over simply smashing everything in sight. Because of this, a common drow priestess rite of initiation is partaking in the summoning of a glabrezu... and then letting it screw her. The half-fiends born of this distinctive union are known as draegloths.
BECMI referred to it as the Howling Demon instead, and it debuted in the Immortal Rules Set (DM's Guide to Immortals), with a weaker "Howling Lesser Fiend" being printed in the Wrath of the Immortals boxed set, in "Book One: Codex of the Immortals".
The glabrezu finally gained its name when the Satanic Panic saw the creation of the Tanar'ri and Baatezu for AD&D 2nd edition. Under its now-iconic moniker, it first debuted (now classed as a "True Tanar'ri") in the Monstrous Compendium Volume Outer Planes Appendix, followed by the Planescape Monstrous Compendium Appendix, from which it was reprinted for the Monstrous Manual.
It finally found its place in Dungeons & Dragons 3rd Edition, debuting in the Monster Manual as part of the Demon super-entry. And in this format it has appeared ever since, returning in both Dungeons & Dragons 4th Edition and Dungeons & Dragons 5th Edition. Being OGL content, they appear largely unchanged in Pathfinder.