Dog Man: The Bark Knight Cometh

Dog Man: The Bark Knight Cometh is a book made by Old George Beard and Old Harold Hutchins in Captain Underpants and the Sensational Saga of Sir Stinks-A-Lot.

Summary
Petey tries to kill Chief and Milly, but Dog Man, as the Bark Knight, tries to stop Petey. Petey promptly reveals himself to be Mecha Petey. He attempts to destroy the Bark Knight, but Milly knocks off his mask, revealing him to be Porkbelly all this time.