The super rig is a gnomish invention of some kind. Its blueprints were locked away in a safe guarded by Mekgineer Thermaplugg in Gnomeregan and whose combination was only known by Thermaplugg himself. According to the goblin Nogg in Orgrimmar, the rig was "better than all our shredder models combined", and he wanted the blueprints in order to figure out all of the gnomish innovations that went into its creation so they could be used for new goblin shredders. It is unknown if by "our" he meant all goblins or just those in service to the Horde.
As discovered by Kernobee, the Dark Iron dwarves were somehow involved in the super rig's creation. For his part, Nogg posited that the gnomes stole the rig from goblins in the first place. However, upon being informed of the presence of Dark Irons in Gnomeregan, he suggested that the gnomes might've stolen the plans for Dark Iron stone golems and modified them, and that the Dark Irons would want payback for this. He later added that this seemed like a reasonable lead as to how the gnomes "suddenly were struck with goblin-like inspiration".
It is never actually revealed specifically what this rig is used for, but because it is compared to shredders and golems it can be assumed that this is some sort of pilotable mech (steam armor). Since the blueprints were guarded by Thermaplugg, its creation involved the Dark Irons (whom Thermaplugg was in contact with, as seen by the Dark Iron Ambassador) in one way or another, and Nogg implies that the rig was only created after Gnomeregan fell and Thermaplugg took control of the city, it is likely that the super rig is the pounder robot type that Thermaplugg drives in Gnomeregan.