in the episode rouge rabbit, perry the platypus is sent to fight an evil rabbit named dennis who is more dangerous than dr. heinz doofenshmirtz is. in the episode “oh, there you are perry”, perry is told to leave phineas and ferb to fight a new supervillain the regurgitator who takes on dr. doofenshmirtz as a sidekick and loses to perry because of doof’s stupidity. in some episodes, perry actually works with dr. doofenshmirtz against a more dangerous villain.