'The Monster of Doom' was a weapon created by Baxter Stockman specifically for the destruction of Dogpound, Fishface and the Turtles. Basically, the Monster of Doom was just an upgraded Stockman-Pod Suit that was programmed with Artificial Intelligence. It's only appearance is in the episode Baxter's Gambit.
It's first and last appearances are in Baxter's Gambit. The monster is at the very center of the Maze Of Doom. It nearly overwhelms Dogpound, Fishface and the Turtles, who are forced to cooperate so that they can escape from Baxter's maze with their lives. Raphael is able to destroy the floating piece of Technology that Baxter is standing on, which causes it to fall and collide with the Monster of Doom, thus destroying it. However, Baxter is able to survive all of this.
- It is unknown where Baxter could have gotten something nearly identical to a T-Pod to make this monster function. Something similar to a T-Pod could be seen in the center of the Monster of Doom. It's not possible that Baxter used the same one that was seen in I Think His Name is Baxter Stockman, as that one got destroyed by Leo.