From Diablo Wiki
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.
The brainstorming and work that went into creating this sequence was discussed between [Bashiok]], [[Julian Love]], and [[Mike Nicholson]] during Blizzcast #8, in March 2009. [http://www.diii.net/blog/comments/blizzcast-8-live-with-diablo-iii-goodies]
::'''Julian: '''Oh yeah, in fact I think there’s been a lot of off-shoot ideas. Like every other monster in the game, where applicable, we ended building a skeletal structure into him, and even a muscular set and things like that, that you see. And those things are functional, so I wouldn’t be surprised if you see skeletal and muscular thousand pounders running around, maybe half-summoned, and that sort of thing. There’s a lot of leeway with it.