Ditto was one of the original Pokémon introduced late into Pokémon GO. The iconic Transform Pokémon no longer just appears as random Pidgey and Ratatta, but rather a whole host of Pokémon. Magikarp doesn’t turn into Ditto anymore ever since Niantic introduced the Shiny version of it. Here’s the current list of Pokémon that may reveal their true form as Ditto:
Catching a Ditto is one of the tasks players must do in order to find Mew, so the above list might come in handy.