Some TMNT stuff really isn't for little kids.
Created by Null Group's mutagen laboratory, the Mutagen Man appears in the IDW continuity as a failed attempt at combining several different animal breeds into one mutant. With his body unable to hold its shape, he was confined inside a liquid-filled prosthetic body which also granted him the ability to move. He was held in a facility by the Null Group, until Old Hob and the Mutanimals find him strapped to a bed, and free him. Although he is told that he's being saved, Mutagen Man has a different idea of the term—by forcing Old Hob's gun to his dome and urging him to fire. Hob refuses and they all escape the facility together, with Mutagen Man becoming a member of the team. Later, Mondo Gecko gives Mutagen Man a more personal name, Seymour Gutz, which he accepts as his only legitimate name.
Seymour proved to be very timid and uncertain at first, since his entire life had been spent as a tormented laboratory experiment. However, he proved himself to be extremely brave, as he rigged his protective suit to explode before heading back to the Null Group's laboratories, so that they could not experiment on other innocent mutants. His plan was unsuccessful, and he went back to live with the Mutanimals, where he continued to make progress with forming friendships. He also became friends with Michelangelo during the latter's brief tenure with the Mutanimals, due to Michelangelo's outgoing and kindly nature. He later met Michelangelo's brothers during the Christmas party.
However, Seymour's peaceful life was soon disrupted. In Desperate Measures, Agent Bishop sent a brainwashed Slash to attack his old friends, and when Seymour was trying to reason with him, Slash smashed his protective dome. He was barely saved by the efforts of Lindsey Baker and the EPF scientists, and his prosthetic body was destroyed.
Some weeks later in Mutagen Maintenance, Seymour was still bedridden in his new, much smaller immobile tank, though the Mutanimals and Lindsey were doing their best to keep his spirits up and construct new limbs for him.
After even more time, in ...And Out Came the Reptiles, Seymour was not only still bedridden in his tank, but had become painfully terminally ill without the necessary technology to sustain his life. Mondo promised to personally scrounge the technology Lindsey needed to rebuild Seymour's support systems and extend his life, but Seymour angrily objected, demanding he be allowed to die and no longer exist as a terrible creature. Lindsey put Seymour into a medically-induced coma to buy some more time to acquire the technology. Mondo requested Hob authorize a Mutanimals mission to raid Earth Protection Force Headquarters; though Hob sympathized with the situation, he thought the mission too dangerous to the rest of the Mutanimals and rejected Mondo's request. Determined to save Seymour's life even over the objections of Hob and Seymour himself, Mondo reached out to Michelangelo for help; after interrogating an EPF employee on how to get into their headquarters, Mondo and Mikey infiltrated EPF headquarters to gather the components Lindsey needed, surviving a confrontation with Agent Bishop in the process. They succeeded, and Lindsey was able to build a new prosthetic body for Seymour, but Seymour hated Mondo for selfishly forcing him to keep living. Mondo confessed he couldn't imagine a life without Seymour as a brother to him, and started breaking down into tears. Seeing Mondo cry also made Seymour start crying. The two hugged, with Mondo hoping Seymour would someday forgive him.