Hamsterdactyls

The Hamsterdactyls (consisting of Tony, Orlando, and Dawn) are the pets of George Beard, Harold Hutchins, Yesterday George Beard, and Yesterday Harold Hutchins. They are also the children of Crackers and Sulu.

They have hamster/Quetzalcoatlus voices in the Sound-O-Rama Audiobooks.

Captain Underpants and the Tyrannical Retaliation of the Turbo Toilet 2000
The Hamsterdactyls hatched from their eggs when the Turbo Toilet 2000 knocked the treehouse again and again. They took a liking to Yesterday George and Yesterday Harold. However, they realized that the boys were in danger and destroyed the Turbo Toilet 2000.

Captain Underpants and the Sensational Saga of Sir Stinks-A-Lot
When Sir Stinks-A-Lot extracted the effects of the 3-D Hypno-Ring and super powers from Mr. Krupp, Captain Underpants was also erased in the process. Old George Beard and Old Harold Hutchins tried to contact their younger selves telepathically; however, their younger selves were too busy sleeping to notice them. The Hamsterdactyls responded and went to battle Sir Stinks-A-Lot, putting a stop to his madness in the city. Later, the Hamsterdactyls joined the present George and Harold in their time travel adventure, hoping to save Crackers and Sulu from their deaths.