Insectosaurus is the overall pentagonist of the Monsters vs. Aliens franchise. He is one of the two pentagonists (alongside General Monger) of Monsters vs. Aliens, and the tetartagonist of Monsters vs. Aliens: Mutant Pumpkins from OuterSpace. He is a parody of Godzilla and Mothra. He was once a little caterpillar who was turned into a gigantic insectoid monster by nuclear radiation and attacked Tokyo.
Insectosaurus originally was nothing more than a simple grub with no unique characteristics, but after his mutation he donned an almost mammalian-like appearance similar to that of an orange hamster with a massive overbite, four pincer-like arms, antennas, two stubby feet, and a short reptilian-like tail. Insectosaurus is a massive creature in size even dwarfing the giant Ginormica. He is seen in the movie using his powerful pincers to pull the bridge back in place in the Golden Gate fight scene. Not only can he do that, he can also shoot sticky web substances from his nostrils that were used only on the Alien Robots eye to stop it from reacting for a short time so the other Monsters could get him and Ginormica (Susan Murphy) to battle Monster on the Golden Gate Bridge. Although of her big physique, he is actually a baby, but that is not approved other than he cannot talk. When he walks, he does that in a dinosaur way and is very slow to walk. Sometimes he doesn't know what to do so he is mesmerized and navigated by a helicopter carrying a light in front of Insecto causing him to stare at it and follow where it goes, also it helps keeps him stable and be amazed at the sight. When that happens, the pupils of his eyes grow almost as big as the size of her eye making him look like a big scary looking monster, but really he is the cutest thing in the film! He cannot hear what the other monsters, aliens and humans are saying (Apart from the Missing Link) so that makes him an (almost) useless, powerful, fun, cuddly Monster.
After his metamorphosis near the end of the film, Insectosaurus' appearance is changed to that of a more butterfly-like creature (similar to Mothra of Godzilla fame) while still possessing his hamster-like face, furry body, massive overbite and reptilian-like tail. He's been renamed to Butterflysaurus.
- For some inexplicable reason, Insecto never appeared in the TV series, nor was he even mentioned.