If you were looking for the 40k vehicle, see Hellhound Tank.
The Hellhound is a monstrous creature from mythology, taking the form of a massive wolf or dog with connections to demons or devils (assuming there's a difference). It frequently appears in various tabletop games as a relatively low-tier extraplanar monster, often with multiple heads and/or the ability to vomit fire over anybody it doesn't like. In Dungeons & Dragons, they're one of the oldest and most recognizable "infernal beasts", far more than their counterparts the Hellcats, and noted for being extremely evil but pack-orientated, and also for spewing fire everywhere. In Pathfinder, you have both the fire-breathing hellhounds and the Cerberus, a bigger, nastier variant with multiple heads, no fire breath and no skin.