Hell hounds are the canine companions of the servants of the denizens of Hell. Bred for size, viciousness and trainability, hell hounds can grow large enough to be used as mounts in the eternal wars between Hell and the Abyss. Wild hell hounds can also be found in wandering in packs throughout this evil plane, and while they're scrawnier than their tamed bretheren, they're also much more cunning.
Evil wizards and warlords will often summon or otherwise procure a hellhound for their own destructive needs. However, the hound's fiery breath and sulphurous smell makes them extremely hazardous to the very flammable Material Plane. Summoning, though only short-term, is probably the more sensible option if you have the means. Otherwise be ready to build stone pens. And make sure that you use good lucks; though beastly, hell hounds are smart enough to work out simpler locks.
Other wizards, fervent seekers of higher truths, also summon hell hounds in an attempt to answer a frustrating question:
Can this hound be a Good Boy when they are, by definition, a Very Bad Dog?