Think about it, Mew and Ditto are the same weight; 8.8 pounds. They are also the same color and they have the same shiny color, too. Mew and Ditto know Transform. They can both learn every move in Pokémon, but in Ditto's case, only for a certain amount of PP. Ditto is mostly spotted in the Cinnibar Island Mansion where the Mew and Mewtwo experiments took place. Mew's experiment went wrong and it came out as the new Pokémon, Ditto.
Ditto is Mew, but its molecules are all scattered so that's why it is shorter and wider, as well as having no bone structure. Mew is said to contain every Pokémon's DNA and Ditto is said it can breed with every single Pokémon, with very few exceptions outside of Legendary Pokémon. If you watched the movie Pokémon The Movie I: Mewtwo Strikes Back, it said there was a failed experiment in the beginning. Now that I look back in it, it could have been Ditto...