The Mutant Burger is as you guessed, a Mutant Burger. It was created by unknown ingredients when the Radioactive Uber Clan tried to make a hamburger. Instead of being eaten by a person, it ended up eating the person who was trying to eat it.
It went to Steve and later tried to eat him. Steve however, calmed the Mutant Burger down and taught it communism. Now it roams around Steves house like a friendly puppy.