Thousand Pounder is a special boss monster, seen in the WWI 2008 gameplay movie and a few related screenshots. It's not known if he's one particular individual out of a whole class of big, fat, dual-wielding, tattooed, demons, or if he's a special, one-of-a-kind demon. He's not an act boss or a major quest boss though, that much was made clear in the panels at the WWI event.
Summoning Ritual
Thousand Pounder is summoned up in a scripted event. The Barbarian in the gameplay demo is running through the dungeon, when he comes upon a circle of five Dark Cultists channeling their energy to call up a demon. At first there are three pale-skinned youths dancing in the circle. They explode one by one, and then from their blood and flesh the demon is conjured.
While the Barbarian watches, lights began to glow and the Thousand Pounder takes form. First his bones appear, then muscles, then skin over the top, until with a final explosion that sends the Cultists flying, Thousand Pounder bursts into life.
Once he's ready to rumble, Thousand Pounder moves slowly, but with tremendous power, and deals heavy damage with his two large maces. When his health drops below about 50%, he kicks into a turbo mode. At that point his tattoos begin to glow red and he moves much more quickly. When he is defeated his death animation is basically the summoning animation in reverse; his skin sloughs off, then his muscles, before his bones clatter to the ground.