Totodile and Feraligatr are not all that different in appearance, but Croconaw, with its egg torso and short, stubby limbs, seems like something else entirely. Pokemon professors have thought up new hypotheses to explain the mystery of Croconaw every decade, some comparing its body to the shells of other water-types, others to the cocoons of many bug and dragon-type pokemon; none, however, have survived peer review. And where logic and experiment has failed, legend has once again stepped in.
Totodile, it is said, despise being born. From the moment curiosity leads them to break their eggshells, they are horrified by the chaos and struggle to survive of the outside world. They live like eggs even once hatched, rarely moving or speaking, doing the bare minimum to survive, until some other frustrated pokemon – in some versions a God speaking to the first Totodile ever born - asks why they even bothered to hatch at all. At first the suggestion offends them, but in time it gains a powerful appeal. And then, like Bagon jumping off cliffs to fly, they evolve through sheer willpower and turn half their body into an egg! In time, however, they will grow bored of an egg's life, make peace with the world of the born, and hatch for a second time into Feraligatr.
This is of course a ludicrous explanation, but that has not stopped parents from telling it to their children for generations, nor has it prevented a sizable minority of trainers from asking this very question of their pokemon in the hopes of speeding up its evolution.