Hell hound

Hell hounds (also called flame hounds or charhounds ) are fiery beasts from the Firelands. Shannox serves as the kennel master to these creatures.

These dogs fear nothing and will not stop until their lust for blood is satisfied.

As a companion pet
Smoldering Treat has a chance to drop from Shannox in the Firelands.

As a hunter pet
Hell hounds are tamable by hunters as part of the hound family. They are classed as undead beasts; only undead hunters can tame them by default while hunters of other races must first learn the skill from a Simple Tome of Bone-Binding.

Warcraft Adventures: Lord of the Clans
In the canceled Warcraft Adventures, Orgrim Doomhammer owned several black creatures which were named "hell-hounds" but bore more resemblance to the darkhounds that would later appear in World of Warcraft.

Trivia

 * The name "hellhound" has been used to describe multiple things in Warcraft: the black creatures owned by Orgrim Doomhammer in the non-canon Warcraft Adventures, the "hellhounds" said to be at the feet of the Black Riders in the non-canon RPG, and the Burning Legion's "felhounds" as said by Kel'Thuzad during their first appearance.
 * Hell hounds were formerly classified as beasts or elementals, as befit their origin in the Firelands, but they were reclassified as undead in patch 9.0.1 to allow taming as undead beasts.
 * Demonic fel wolves and undead skeletal hounds reuse the hell hound model but with different textures.