Ditto is one of the most elusive Pokemon in Pokemon Go. It is unique because Ditto doesn't reveal itself until you capture the Pokemon it is disguised as.
Introduced in the Kanto region, Ditto is one of a kind because it takes the form of other Pokemon. Using its transform move, it copies the physical form and moveset of opposing Pokemon. Ditto wasn't available when Pokemon Go was first introduced but found its way into the game shortly after.
Catching a Ditto in Pokemon Go is the same as capturing other Pokemon. It appears randomly like others, the same catch mechanics apply and it is even relatively easy to catch in comparison to rarer Pokemon.
However, the difficulty comes as Ditto appears as other types of Pokemon in the Pokemon Go wild. Moreover, it tends to mimic more common Pokemon, meaning a Ditto can be sat there and you remain completely unaware.
As Ditto is rarer than the Pokemon it appears as, that Pokemon will appear harder to catch than normal.
One way to increase the chances of capturing Ditto is to know the Pokemon it will disguise as. While they are Pokemon that appear frequently, the downside is it is likely you have already caught them. Consequently, you may not bother capturing them.
As of March 2019, Ditto can disguise itself as the following:
Unfortunately Ditto won't reveal itself until you have captured it. Upon capture instead of saying "Gotcha!", a message will appear saying "Oh?", similar to when an egg hatches, and it is then Ditto will appear.
Capturing Ditto is actually one of the research tasks for the "A Mythical Discovery" quest which grants Mew upon completion. Naturally Ditto can be somewhat of a stumbling block.
There isn't a lot more you can do to complete this task, other than continually capturing the above list of Pokemon. Alternatively, you can search the Pokemon Go forums to see if anyone has shared a Ditto location. Search "Ditto Reddit" to find groups in your local area.
The spawn rate of Ditto is unknown but it certainly isn't common. Some Pokemon Go players report that Ditto isn't as infrequent as it is made out to be. On our side it took until level 21 and level 24 to encounter it though.
If Ditto is all too rare for you, our advice is to preserver because eventually it will appear.