Theories About Finding Ditto

Theory From the anime: 

This theory of them all seems to be the most believed of them all. The anime "Ditto's Mysterious Mansion" which is the 36th episode reveals Ditto. Ditto and it's trainer Duplica are masters of disguises. Starting off Ditto is first seen as a Pikachu, though is noticeable different from Ashes because it's eyes and mouth didn't transform. When the 2 trainers battled, Ditto transforms into Ashes Bulbasaur, again with the missing facial features.

Team Rocket, of course, intrudes on the trainers later on in the episode, though they weren't there for Pikachu, they were after Duplica's  Ditto with the intention of forcing Ditto to evolve into a Dragonair. After a few attempts, Team Rockets deems the Pokemon is useless, though transforms into  Meowth, again it's face didn't transform. Team Rocket not happy with that, they threaten the poor thing to make its face look like Meowths, and it worked. 

Ditto transformed into 3 different Pokemon in the Anime, Pikachu, Bulbasaur, and Meowth. Pikachu and Bulbasaur's transformation both had the Ditto facial features, while Meowth's did at first, but later on Ditto finally got the whole transforming technique down. The theory is there will be an encounterable Pokemon with Ditto's face which will then transform back into its original form somehow. Some believe the only Pokemon you will find this from are Pikachu and Bulbasaur, though there hasn't been much talk over Meowth.

Regional Pokemon Theory: 

Many of us know that Ditto is a Mew experiment gone wrong. Ditto then escapes or is released into the jungles of South America. I am unsure if Niantic plans on releasing the game in South America, but if they do then it would make sense why Ditto has a catch rate but has yet to be caught. Making Ditto regional Pokemon overall makes the most sense, mostly because no one has solved the "easter egg" yet and everything or just about everything has been done to try and find this unique Pokemon.

Yes Ditto has been determined and it is said to have a catch rate, doesn't mean that it is currently catchable. Not knowing what the catch rate is, or the other regional Pokemon's catch rates, they more than likely will be similar. Why can't Ditto be hatched from 5k eggs like the other 4 regional Pokemon? It isn't in the game yet, it would only make sense to give South America a regional exclusive Pokemon, but that can't be done, unless it's Ditto.

Don't know how to implement it:

Another less common theory is, the developers may not know how to put the Pokemon in the game. When you think about it, how would a Ditto work when dueling other players when the feature is added to the game or taking over/defending gyms. The Pokemon of course, would transform and assumption is that Ditto would know the opposing Pokemon's move set. 

There is a glitch in the game now after the recent update, not many have brought to the conclusion could they be testing or could the coding for Ditto's transform somehow got in the game? The glitch happens after catching a Pokemon, you could be catching a Pikachu one second and the next after it's been caught it could be a Weedle, or it could be a Dragonite. The Pokemon "transforms" into something other than the actual Pokemon.