After reading through the thread my guess is that there was indeed a switch but it might have happened deliberately.
as
>>37268658 said, the caterpie line >acts as the perfect paradigm to teach the concept of evolution in pokemon being that its a transformation that also happens in real life and being placed so early on in the game and evolving at such a fast rate its likely the first contact with evolution new players will have.
It seems to me that for this reason they might have wanted to make the players first experience with evolution something cool and let's be real, creature 2) in pic related is not very appealing while creature 1) is. Remember that Pokemon was heavily inspired by Kaijus and before there was Volcarona, creature 1) was pokemon's version of Mothra.
