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 attept 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 "Please kill me! PLEASE!"