From Diablo Wiki
Nothing is specifically known about the background of the Thousand Pounder. Is
he a "[[Superunique]]" of this "Fat Demon" kind, or is it a type of monster all together? The concept art seems to indicate that it's not a unique entity, showing a Barbarian with several of these creatures, but the matter in which he is summoned could suggest he is a little bit of a personality anyway. He is in either way powerful enough to require a quite advanced summoning to enter the physical realm.
==Diablo I & II Monster==