Barfilisk

This barfilisk is the main antagonist of "The Bizarre Bout of the Beastly Barfilisk." Whenever people look into his eyes, they barf a rainbow.

The Epic Tales of Captain Underpants
It is revealed that the rock George and Harold took to make it look like a barfilisk egg was actually a real barfilisk egg. The printed barfilisk is also revealed to be male, and it hates Melvin. The barfilisk then falls in love with the barfilisk mom and starts to help her raise the baby barfilisk.

Trivia

 * The barfilisk is based on the mythical creature, the basilisk.