Professor Nibbles is a one-shot villain in Disney's Phineas and Ferb. He serves as the villain of a fictional show named Katt Karr (created by Phineas Flynn and Ferb Fletcher). He wears a mouse costume and only appears in the episode Ferb TV.
According to the episode, his nemesis (played by Danville Mayor Roger Doofenshmirtz, the younger brother of Dr. Heinz Doofenshmirtz) and a sentient car resembling a cat called Katt Karr are hunting down Professor Nibbles for some unknown crime he commited. As both Roger and Katt Karr catch to up him driving in his motorcycle, Nibbles takes out a ball of yarn and throws it out, knowing that cats can't resist playing a ball of yarn. This distracted Katt Karr to chase after the ball of yarn, much to Roger's anger. However, in the end, Roger and Katt Karr inexplicably manage to stop Nibbles in his tracks and he is sent away to prison.