Developers Niantic have confirmed that Ditto - a little amorphic purple blob - has been added to Pokémon GO. It's not going to be as easy to catch as your everyday Pokémon however, as Ditto has the unique ability to transform into a wide array of other Pokémon, so on the map it will just appear as a Rattata, Pidgey, Spearow or Magikarp. Once captured, his true self will be revealed and he'll be added to your Pokédex.
According to Niantic, his ability to shape shift was the reason Ditto wasn't included in the game from day one, as he proved difficult to code. Watch a video of a Ditto being caught, disguised as a Rattata, below.