Baxter Stockman is a character who dates back to the original “TMNT” comic books. Originally depicted as a Black man, the character is responsible for creating the robots known as Mousers, initially designed to help exterminate rats around New York City. However, Stockman was actually using the Mousers to rob banks, and he was concocting another plan that would allow him to hold the city for ransom. Stockman would recur in the comics as a villain, but his most well-known form wouldn’t come until the 1987 animated series.
Reimagined as a nerdy white scientist, Baxter Stockman still created the Mousers, but his intentions were noble and he actually wanted to help the city. However, Shredder saw the Mousers as a way of getting rid of Splinter, so he replicated them with Stockman’s research. The scientist’s involvement with their invention put him on a path with the Turtles, and he was arrested and committed to an asylum, where he went crazy and actually turned into a villain.
In the second season of the animated series, entitled “Enter the Fly,” the villain Krang no longer needs Stockman’s scientific talents, so he tries to kill him by throwing him into a disintegrator unit. However, trapped in the chamber with him was a common housefly, and rather than being killed, Stockman’s molecules became mixed up with the fly, turning hm into a humanoid fly creature inspired by the movie “The Fly.” Though he initially attacked both the Turtles and Shredder in an attempt for revenge, Shredder twisted his mind and convinced him it was the Turtles who were solely responsible, thus turning him into a full-fledged villain.
Why do we recap all this information? Because Baxter Stockman appears in “Mutant Mayhem” and has a pivotal role in the movie, but despite assumption from fans, he does not turn into the new villain Superfly. That was the intention, however, at one point in the movie’s development, and there’s evidence of that in one of the “TMNT: Mutant Mayhem” toys that made it to shelves.