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.
- 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, although, according to Ciro Nieli's The Art of Teenage Mutant Ninja Turtles, the Squirrelanoids were inspired directly by the Pizza Monsters.