In Pokemon, each individual Pokemon ''evolves'' into a new stage of its evolution. A lot people claim that this can be explained using mere metamorphosis, but there are no mammals that go through such complex metamorphosis'. 

Using earth-like biology how can I explain how so many animal kinds (from canines and felines to giant snakes and land sharks) can undergo  massive and limb growing metamorphosis? Why would evolution favor a creature changing its form so radically?