Ditto is sort of the perfect Pokemon for collectors- it can transform into literally any other Pokemon, down to their moves, and it can also be used to breed just about any Pokemon that there is. It is, therefore, one of the most sought after Pokemon in any Pokemon game- which is why its absence in Pokemon GO until now was such a big deal, and why its addition to it now is such a big deal, too.
Ditto was added to Pokemon GO last night, meaning you can now finally get one for yourself- but it comes with a catch (pun intended). You see, Ditto does not show up as Ditto on the world map. Rather, you must catch the Pokemon that you see on the map, and once you catch them, they can reveal themselves to be Ditto. This revitalizes the game, so to speak, because even a Rattata or Pidgey in the wild could now be an extremely valuable Ditto.
Pokemon GO is available now on iOS and Android smartphones. Its success has galvanized the Pokemon brand, and led to Pokemon Sun/Moon becoming the largest selling games of this year.