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 another Pokémon in their normal form (not Shiny). Ditto will only reveal itself once it has been successfully captured and it may be normal or Shiny.

Currently, the following Pokémon in the wild can be a Ditto in disguise:

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 only one 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, players reported sightings of an undiscovered Pokémon which turned into Ditto once captured, and was later identified as Meltan.
 * Ditto used to appear disguised only as non-Shiny-released species until the Season of Mischief. If a disguise Pokémon happened to release its Shiny form, that Pokémon was removed from the list of Ditto disguises.
 * The following species had previously been possible Ditto impersonations: Weedle, Pidgey, Rattata, Ekans, Zubat, Paras, Venonat, Mankey, Gastly, Drowzee, Voltorb, Magikarp, Sentret, Hoothoot, Ledyba, Spinarak, Natu, Hoppip, Yanma, Teddiursa, Remoraid, Zigzagoon, Seedot, Surskit, Whismur, Skitty, Gulpin, Numel, Bidoof, Stunky, Finneon, Lillipup, Purrloin, Dwebble, Foongus and Swirlix.
 * When Shiny Paras was released, some non-Shiny Ditto disguised as Shiny Paras, but the bug was corrected later.
 * Since Shiny Ditto was released in the wild, Ditto can disguise only as non-Shiny variants, even if the Ditto itself is Shiny upon caught.