Meltan

Meltan is a mythical enigmatic Pokémon that appears only in the wild and, after being caught, turns into Ditto.

Trivia

 * Before its official name was announced by game creators, in game data it is referred either as "???", "Pokémon 891" or "Kecleon", following names were given for this Pokémon in player community, "Nutto", "Nutface", "Incogditto". All of these nicknames refer to its nut-shaped head.
 * Its has a small, silver body resembles Ditto and a hardware nut-shaped head with an eye floating inside, which when displayed in nearby as shadow silhouette resembles Unown.
 * It cannot be rotated in the encounter screen and in the official gameplay, there is no possibility to have it in screens such as Pokémon summary page, Pokédex entry nor Buddy Pokémon screen and in the Map View its model's scale is incredibly small, however using AR+ or third party software to manipulate gameplay its rear can be seen with a little red wire tail visible.
 * In case of shiny variant this tail has a blue color.