Pigzilla was a gigantic pig that falls from the sky in the shape of a comet. It will walk around and destroy everything in its path. Pigzilla made it's appearance in Mod Mod World.
After Tommy and Megan became friends, Megan told Tommy that she wanted to discover a dinosaur mod. She said that she had dinosaur DNA. Tommy had taken a strand of hair and analyzed it. Tommy didn't believe she actually had dinosaur DNA, but when he tested it, he found non-human DNA. They set up everything and started a machine that would uncover a dinosaur mod. However, Megan had bacon in her hair, not dinosaur DNA and it spawned a gigantic pig that destroyed parts of their town.
After destroying a little of their town, Modzilla poured a potion on Pigzilla, causing him to shrink down to the size of a normal pig. Prince Reese and his brother, Prince Marsh, heard about the wreckage of the town (the pig did not damage their home) and Prince Marsh had taken the now normal Pigzilla to their basement for further integration. Aphmau rescued him and brought him back to her place, where she would set him free, but he just followed her inside her house. Marsh came over to fight with Pigzilla, but Aphmau made Pigzilla a dinosaur costume and Marsh didn't recognize him.
Megan came over to Aphmau's house shortly after Marsh left, she thought that Pigzilla was a real dinosaur and took him home with her. She didn't think that her dad would let her keep him, so she had Aphmau build her a secret pig house for Pigzilla, so her dad wouldn't see him (hopefully).