Chunky Cheese Pizza Monsters is the popular name given to the unnamed antagonists of the Turtles, from Dimension X of the 1987 TV series episode The Case of the Killer Pizzas in the instruction manual of the Super NES version of Teenage Mutant Ninja Turtles: Turtles in Time. In the ending credits for the SNES version of Turtles in Time and Back from the Sewers they are simply called Pizza Monster. In the manual for Back from the Sewers they are called Sewer Mutant.
1987 cartoon series
The Pizza Monsters are from Dimension X and are born from meatball-like eggs. Warming them causes these eggs to hatch. As juveniles they are red and very small, but when they come into contact with water they grow quickly to gigantic size and turn yellow. They even have the ability, to burn materials such as metal. Three of the Pizza Monster eggs were brought from Dimension X by Krang and given to Shredder to destroy the Turtles. These were scattered on certain pizzas by Baxter Stockman under orders from the Shredder and given to the Turtles as prizes. But the pizzas fell into the hands of April, Irma and two children, who warmed up the pizzas with a microwave and thus inadvertently helped to hatch the monsters. They were controlled by Stockman with a remote control, but it was lost. Through clever use of the instructions Stockman and Shredder learned that a even larger bust of energy would turn the pizza monster back into eggs again. The warring parties had joined to fight against the monsters. Hearing what Stockman said Leonardo severed a power line, whereupon the monsters were hit with electricity and again assumed their original shape. What happened to them afterwards remains unclear.
2012 Animated Series
While the Pizza Monsters do not occur directly in the 2012 animated series, in the episode "The Mutation Situation" there is a little nod to them. Michelangelo thought up the "Cottage Cheese Demon" which he defeated using a microwave.
Video Games
The Pizza Monsters occur in Teenage Mutant Ninja Turtles: Turtles in Time and Teenage Mutant Ninja Turtles II: Back from the Sewers. In the manuals and credits of the games you can learn their name.
Trivia
The monsters themselves highly resemble yellow versions of the "Xenomorphs" from the sci-fi horror film series Alien. A similar parody is found in the Squirrelanoids from the 2012 TV series.