The Giant Borg was an immense battle suit crafted by unknown aliens in another dimension.
Unlocked through a device known as the Giant's Hand, it was sought after by the first Robo-Robotnik v1.0, who wished to use its power to dominate all the various incarnations of Mobius. Claiming the weapon, he set out on a rampage of Mobius Prime, only to be set upon by an army of alternate Sonic the Hedgehogs. The Giant Borg was fragmented by the battle, and each of the Sonics was given a piece of it to take back to their home dimensions in hopes that it would never be reassembled by any evil power. (StH: #19)
Years later by his universe's time, Robo-Robotnik (now reduced to a programmed consciousness on a derelict space station) found himself discussing his current predicament with Dr. Robotnik, the latter having been accidentally transported there by his rogue creation E.V.E.. When he returned him to Mobius Prime, Robo-Robotnik secretly stole location of the Giant Borg piece in his own dimension from Robotnik's memory. Now aware of its location, Robo-Robotnik managed to acquire the Giant Borg piece and construct a new robotic body for himself, ending his restriction to the space station. (StH: #21, #22, #75)
Not long after this, Robo-Robotnik recruited Evil Sonic to help him recover the remaining scattered components of the Giant Borg, only for Evil Sonic to mistake the Dr. Robotnik of the Sonic Underground universe for him. The Underground version of Robotnik re-assembled the Giant Borg and piloted it in an attack on his citizens, only for Sonic Prime and that zone's Sonic to destroy it by luring one of the suit's own missiles back towards it. (SSS: #10)