Ditto

There's a certain list of pokemon that have a chance to be Ditto in hiding. Rattata, Whismur, Yanma, ect. You can find them by searching the list that could be Ditto.

There is no way to tell that the pokemon is Ditto until it reveals itself after being caught so you gotta catch them all.

Currently there are no Ditto-rewarding tasks, so its down to pure luck. Or, in my experience, not so much luck but malice. I once caught two Dittos within five minutes and the very next Pokestop did give me a Ditto quest. At a later occasion I had just deleted a Ditto quest in frustration when they did the Mealtan teaser. Recently, a co-raider told me he had been searching Ditto for a week already - two minutes after we parted, I caught one (I telegrammed him and he came back, preferring to miss his train over missing the Ditto).

If a Pidgey, Rattata or Hoothoot runs from you, it was probably a Ditto