Pokémon GO fan site "The Silph Road" has been rather busy tearing through the game's .apk files and dug on some hot information. It looks like 2nd Gen Pokémon and Ditto are currently baked into the game, and are just waiting for Niantic to finish testing and flip the switch to release them.
Pokémon's 2nd Generation will introduce the 100 Johto region bunch to the game. Just remember not to play and drive while looking for the new elusive creatures if they launch.
This will range from Pokémon Gold to Pokémon Silver. This introduces Pokémon starting at #152 (Chikorita) to the famously and adoringly loved Celebi (#251) to the game. This means the guy who caught all, but a select few that just happened to be regionally exclusive, will once more go on his beloved hunt to "Catch 'em All."
According to the report it very well seems that Ditto may even be joining the line up sometime soon. This also includes the fact Ditto can use his infamous signature move, Transform, since it appeared in the games .apk file. However, he's been existent in the .apk file before today, dating clear back into July. Sadly, he's never been caught, and if he can be, he's as elusive as ever. Perhaps he's disguising himself as that Pikachu that you so desperately hunted down, who knows, right?
Well, at least logging in will reward you nicely if you decide to play every day for a few daily quest rewards. Just remember, make sure to dress up as your best trainer possible whether it's Halloween or not.
Source: The Silph Road