Dennis the Bunny, nicknamed "Mr. Cutie Patootie" by Candace, is a rabbit and a former agent of the O.W.C.A. who went rogue and attempted to hack into the mainframe of the organization. According to Major Monogram, he is a master of disguise, proven by several images of Dennis that are sinister, followed by his harmless "mindless animal" look.
In order to gain access to the Flynn-Fletcher household and, by extension, Perry's Lair and the O.W.C.A. mainframe, he lets Candace see him. She immediately attempted to adopt him. Candace used him as a living doll for dress-up and taught him a couple of tricks, inadvertently showing his obsession with carrots. Linda spots Perry and Dennis battling, but on her double-take, sees both posing as mindless animals. She seems unconcerned that Candace has adopted a pet without her consent. He was defeated by Agent P, whose lair he had infiltrated to hack into the organization, and likely taken into captivity by the O.W.C.A. ("No More Bunny Business").
- Like many rabbits, Dennis has a strong fondness for carrots and has even interrupted his mission in order to eat carrots, which eventually leads to his capture. He eats so many carrots he cannot fight back when Agent P handcuffs him.
- Dennis growls as his only vocalization, reminiscent of Agent P's own chatter.
- Carl dresses up as him while he tries on disguises ("Undercover Carl") but there is a color error because he is mistakenly pink.
- It could be that he is an allusion to Alec Trevelyan from the James Bond movie Goldeneye.
- He also has a "mindless-pet mode" like Perry and Pinky.
- He was mentioned by Candace in the game based on Phineas and Ferb the Movie: Across the Second Dimension.
- He is one of the only known agents to have a name that is not an alliteration, such as Perry the Platypus, or Peter the Panda. Pinky the Chihuahua is not alliterative either.
- He can dance very well, being able to do "the Worm" and "the Robot" ("No More Bunny Business").
- He returns for a second time in the series ("The Return of the Rogue Rabbit").
- Like Perry and Pinky, Dennis doesn't "talk."