One of the greater "Pokemon Go" mysteries, as Niantic has declared that players have yet to uncover all the Easter Eggs hidden in the game, is whether Ditto is already in; and if he is, where could he be found? People have tried to look for the form-changing Pokemon everywhere, but to no avail.
A Redditor, however, encountered an incident in "Pokemon Go" that may either be Ditto, lead to Ditto, or just simply be a visual bug in the game. Redditor dyazge says that he caught a Caterpie with an unusually high combat point stat (the stat that determines how a Pokemon fares during a battle). When he eventually checked it in his storage, the Caterpie turned into a Charizard. Other players have reported similar things happening to them, like another player catching a Magikarp that turned into a Dratini.
If it were a Ditto in "Pokemon Go," however, it should have transformed back into a Ditto already, or it would have had the telltale Ditto face. Or something. Not to dash the hopes of players who want a Ditto, but if we were getting the Pokemon that easily, we would've already gotten it by now in its true form. It most likely is just a quirky bug in the game that causes some Pokemon to look like others.
This isn't the only bug "Pokemon Go" players have had to encounter during the short time the game has been out. The biggest frustration they've had to deal with right now is the removal of the nearby Pokemon tracking system, and the nuking of third-party apps that enable better tracking. Others have devised workarounds around this (such as an Android smartwatch app), but we don't expect them to be long-lived knowing the way Niantic goes after these things.