Pokémon X and Pokémon Y

afaik mew is just the "original creation" of arceus and is able to learn all techniques because of that. but i'm quite foggy on poke-lore currently.

I was being sloppy but essentially Ditto is the failed duplication of mew. They are essentially the same color, they are the same weight, they are the only two pokemon in the universe to learn transform, and in the original series ditto were only found in cinnibar island, where mewtwo was created.