It first appeared in I Think His Name is Baxter Stockman when the T-Pod upgraded Baxter's mechanical suit into a bigger threat to the turtles and in its second appearance the suit is called "Monster of Doom" and has its own A.I. based on Baxter.
The first version was originally a rather crude design, but was the best thing Stockman could build with the technology available to him: An unarmored exoskeleton with a welding torch mounted on the left forearm. The weaknesses of the suit lay in its lack of a decent armor and armament, and an insufficient energy source consisting of two car batteries carried on the back. Before being affected by the T-Pod the suit was able to enhance Baxter's strength.
Second form "Stockman-Pod (Mark 2)"
Through the influence of the T-Pod, which was equipped with the latest military technology, Stockmans exoskeleton was transformed into a construct of living metal. The further the T-Pod was connected with the suit, the more skills were available to the suit with time, especially the spontaneous development of newly incorporated weapons systems. Besides enhancing the user's strength the suit now could release its claw hands and use them as drones, which had lasers.
Third Form "Stockman-Pod (Mark 3)"
The third version of the Stockman-Pod was not as technically sophisticated as the second, nevertheless it was still far superior to the Turtles, in open battle. After its second upgrade the suit was able to shoot missiles. However, this version was not completely armored; Stockman had left an open viewport at the front of the suit, which Michelangelo clogged with a wasps nest and thus caused Stockman to quickly lose control of the suit.
This version of the suit appears in the Baxter Robot Rampage LEGO set.
Final Form "Monster of Doom"
After its last upgrade Baxter had it improved for Baxter's Gambit, adding lasers, M.O.U.S.E.R.S. and an artificial intelligence. In contrast, this suit was not controlled by Stockman directly from the inside, but with the help of a helmet and Stockman's thought pulses. However, this was of the greatest weakness of the Monster of Doom, because Stockman was located outside of the suit in this version and thus was vulnerable to attack.