Dennis the Rabbit is a recurring antagonist of Phineas and Ferb, serving as the main antagonist in the Phineas and Ferb episodes, "No More Bunny Business" and "The Return of the Rogue Rabbit". He is a former rabbit agent of The O.W.C.A. who went rogue and attempted to hack into the mainframe of the organization.
No More Bunny Business
Dennis first appeared in "No More Bunny Business", where Major Monogram tells Agent P that he is a master of disguise, showing several images of Dennis that are sinister, followed by his harmless "mindless animal" look. In order to gain access to the Flynn-Fletcher household he lets Candace see him. She immediately attempted to adopt him. Candace used him as a living doll for dress-up and taught him tricks, inadvertently showing his obsession with carrots and foreshadowing his true nature. Linda spots Perry and Dennis battling, but on her double-take, sees both posing as mindless animals.
After Linda leaves, Perry and Dennis resume their fight against each other, eventually revealing Dennis for what he really is: a feral and dog-like savage who is convinced that he is really a dog and will do anything to accomplish his goals even if it means murdering agents to which he had previously attempted to do but decided not to and ultimately taking it down to Perry's lair, where Dennis manages to capture Perry. He then takes Perry's fedora hat and uses it to help hack into the O.W.C.A. mainframe. However, he is interrupted by a carrot that fell down into the lair, and starts eating.
Taking the opportunity, Perry manages to get a wide-load of carrots to fall into his lair, distracting Dennis for Perry to finally escape from the trap and put Dennis in handcuffs before reclaiming his hat. It is implied that Dennis would later be sent to prison afterwards.
The Return of the Rogue Rabbit
Dennis returned again in "The Return of the Rogue Rabbit", where he is first seen locked up in his cell. However, he is soon freed by Norm, the robot assistant of Perry's archnemesis, Dr. Heinz Doofenshmirtz. After bringing Dennis back to his lair, Doofenshmirtz informs that he is perfectly aware of Dennis' occupation as a mercenary and can't be fooled by Dennis' 'mindless animal' mode.
Doofenshmirtz then offers Dennis a deal: he will provide Dennis technology to help him beat Perry, and in return, Doofenshmirtz will get to take over the Tri-State Area. Seeing this as an opportunity for revenge, Dennis eagerly accepts the deal. To start off, Doofenshmirtz equips Dennis with a backpack that springs out an armourer suit designed for combat. Taking delight in this, Dennis flies off to battle against Perry, ignoring Doof's orders on how to use the armor as he wanted vengeance on Perry for exposing his true colors and decides to murder him in cold blood. During their battle, they were almost found out by Candace, who recognizes Dennis as the rabbit she tended to adopt.
After Candace leaves, Perry and Dennis resume their fight again, this time, Perry uses the Fireside Girls' marionette to combat against Dennis. This was too difficult for Dennis to deal with as he ends up being beaten by Perry. After Perry defeated Doofenshmirtz and send him drifting into the water with the giant marionette, Dennis attempts to escape, but is met by Candace, who is about to take him home. However, a disguised Carl and Major Monogram arrive, telling the rabbit is their 'pet', Mr. Bigelow. Deciding that it's not worth it, Candace gives Dennis back to Carl and Monogram, who then secretly send Dennis back to prison.