In Phineas & Ferb Universe apparently exist two types of animals (not including humans), animals that work in O.W.C.A and "normal" ones.
What I don't understand is, how animals that work in O.W.C.A became what they are. They can understand human language, they can fight and just overall more intelligent, than other animals. Not to mention that they also can grab stuff with their hands and stand upright.
For example, there was an episode where Phineas and Ferb invented animal language translator. Normal animals can not understand human language without it, but special ones like Perry, can.