Ditto

Ditto is a Normal-type Pokémon from the Kanto region. It does not evolve into or from any other Pokémon.

Pokémon disguises
Ditto cannot be seen in the map view, because it disguises itself as other Pokémon. Ditto will only reveal itself once it has been successfully captured.

Ditto can disguise itself in wild as following Pokémon:

In past, Ditto used to disguise itself in wild as Weedle, Pidgey, Rattata, Zubat, Paras, Venonat, Mankey, Gastly, Voltorb, Magikarp, Sentret, Ledyba, Yanma, Zigzagoon, Seedot, Whismur, Skitty and Bidoof, but after the release of their shiny variants, Ditto can no longer be disguised as them anymore.

Trivia

 * Ditto is only non-Legendary or Mythical Pokémon from the Kanto region that was not available with release of the game, as it possesses special move - Transform.
 * Ditto can only transform once per battle. It will retain its first transformation for the duration of the battle.
 * Ditto can only be used in Gym, Raid Battle and against Team GO Rocket, but it is banned in Trainer Battle, including training battles with Team Leaders.
 * However, Ditto's Transform does not work when fighting Team GO Rocket. This is likely an oversight as a result of Team Go Rocket battles being similar to Trainer Battles, which it is banned in.
 * Following the conclusion of September 2018 Community Day event, players reported sightings of an undiscovered Pokémon which turned into Ditto once captured, and was later identified as Meltan.