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.
During the chase as both Roger and Katt Karr catch up to him driving in his motorcycle, Nibbles takes out a ball of yarn and cackles evilly as he 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 to let Nibbles speed off more, much to Roger's anger and frustration.
However, in the end, Roger and Katt Karr inexplicably manage to stop Nibbles, who ends up being restrained by Roger while Katt Karr coughs up a huge hairball from unknown causes, much to both Roger and Nibbles' horror. It is implied that Nibbles is sent away to prison for his crime.