Mews are ‘un-differentiated’ Pokémon
Ok so this is a pretty nifty theory on everyone’s favourite cute pink cat/mouse hybrid… thing. So the theory suggests that Arceus is the God Pokémon and, after he created time and space, with Dialga and Palkia to govern it, he wanted to populate said planet.
So what does he do? Sends a plague of Mews… sure as hell makes a plague of Locusts look a bit shit! The theory argues that over time, the Mews started to differentiate depending on the environment they found themselves in. Tada! You have Pokémon with different elements and different characteristics to help them adapt to their particular surroundings.
Ah! You say, but Mew has the DNA for all Pokémon. Correct! But the theory argues that once it differentiates to suit its environment, it loses everything it doesn’t need and so you end up with Pokémon remaining the same, not flicking and switching between different Pokémon.
Unless you’re talking about Ditto of course, but the whole ‘Mew= Ditto’ theory is discussed in the comments for the theory. You can check out the theory and the developments forming in the comments here.