Ditto

How to Catch Ditto
Ditto appears to be hiding as one of several low-tier pokemon. These include Pidgey, Rattata, Zubat, and Magikarp - but may include others.

Once the masquerading Ditto has been caught, an "Oh?" will appear above the pokeball - not unlike a hatching egg - and in a cutscene, the fake pokemon will switch forms and you will find that it was Ditto.

Attacks
Unlike other Pokemon, Ditto only has one Transform move (plus Struggle). Using Transform in a Gym battle reportedly turns Ditto into a mimic of the opposing Pokemon - complete with the same moves and CP level.

Basic attacks

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Special attacks

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Trivia

 * As of November 22nd, Ditto was released in the wild.
 * Defensive Dittos cannot not transform again during battle as it will copy the move of the first Pokémon it transformed into.