A number of changes have been pushed out to Pokemon Go in recent months following the explosion in popularity the game enjoyed in mid-2016.
In addition to switching on the nearby feature in some regions, one of the unexpected changes was the inclusion of Ditto, one of the original gen 1 Pokemon that wasn't featured in the game at launch.
Ditto, as the name suggests, can copy the form of other Pokemon. In its natural form Ditto is a purple blob, but when you find Ditto in the game it will be in the form of something else, like a Rattata or Pidgey.
You won't actually know that it's Ditto until you capture that Pokemon. After a successful Poke Ball throw, rather than saying "Gotcha", the "Oh?" text will come up - just as when you hatch an egg.
Then that original common Pokemon will transform itself into Ditto and add itself to your Pokedex.
Obviously, if you've been playing for some time and ignore many of the common Pokemon, you'll now have to catch some of them to get yourself Ditto - which I a clever way to get people playing more widely and not just hunting for rare Pokemon types.
Ditto is said to hang-out near people and settlements, which might help you track it down, although it's not especially good once you have it, as it's a weak fighter, unable to replicate the strengths of the Pokemon it mimics.
Ditto appears in the game as there's a lot of talk about an expanded set of Pokemon being introduced to Pokemon Go to bring back those players who have either caught them all, or got bored of finding the same critters.