Appearing in multiple incarnations of the Teenage Mutant Ninja Turtles franchise, Baxter Stockman is a classic TMNT villain. He is a scientist who invents the sewer rat-hunting machines, the Mousers, for a pest control company, but they are deemed too efficient at killing and therefore too dangerous. Stockman is then hired by The Shredder to destroy the Turtles and Splinter, which he attempts time and time again using reprogrammed Mousers.
In most TMNT media, including the original Mirage comics, Baxter is portrayed as an African-American. However, in the first animated series and games derived from it, he is Caucasian.
In the original TMNT Animated series, Shredder had had enough of Baxter's failures and decided to punish him via disintegration. Baxter was then taken to the disintegration chamber, where unbeknownst to him and the others, a common housefly flew into the chamber with him. The disintegration process results in an error that causes Baxter to fuse with the fly, turning him into a mutant fly hybrid. In his video game appearances, he is most commonly seen in his mutant fly form. Stockman also becomes a mutant fly in the 2012 animated series and associated video games.
In the original Mirage comics as well as the 2003 series, Baxter becomes a cyborg. In the comic, this is of his own accord; he has his brain removed from his human body and inserted into a robot body. In the cartoon, he is continuously punished by The Shredder, whittling down body parts. He eventually places himself into a giant mecha body, which is destroyed, and he goes through a series of cyborg bodies until he is nothing more than a brain, brain stem, and eyeball floating in a tube. He eventually does obtain more cyborg bodies to house this new form.
In Video Games
Baxter appears first in the arcade game Teenage Mutant Ninja Turtles. He appears in a flying contraption, dropping Mousers to vex the player. In the NES version of the same game, he reappears in his mutant fly form (replacing the Bebop & Rocksteady dual boss fight from the arcade version). The instruction manual gives his mutation the nickname "Knat Man".