In the episode Of Rats and Men, the Rat King planned to turn every human into a mutant rat person. The Rat Man Freak was his first attempt to do so, mutated using an untested mutagen serum. Unfortunately, the serum wasn't ready yet, so the man he tested it on turned into a hideous, disabled monstrosity.
His name is not mentioned in the show, but comes from the credits, where it is divulged that he is voiced by Jason Biggs.
His only line was "Cheese! Please give me CHEESE!"
Rat Man Freak was Rat King's first attempt to make rat people, humans mutated into rats just like Splinter, but with the ability of infecting other people with their rat mutation so Rat King's kingdom would spread and grow at every bite. But of course, his utopia plans were frustrated by Splinter and the turtles. Irma was going to be his second experiment.