Ditto is finally catchable in Pokemon Go, according to multiple reports online. It appears the transforming blob is masquerading as other Pokemon, and only reveals itself after you catch them.
Some users seem to think this phenomenon is restricted to, or more common in, critters in the 2 km egg group--like Spearow, Magikarp, and the generation one starter Pokemon. Others have reported Ditto is emerging from rarer monsters, though everything's a little uncertain right now.
In battle, Ditto acts much like it does in mainline Pokemon games: it only knows one move, Transform, which metamorphoses it into your opponent's Pokemon, simultaneously adopting that enemy's moves.
Pokemon Go developer Niantic has yet to officially confirm Ditto's existence. The company recently said Go was still hiding at least one secret, shortly after Ditto's possible introduction was uncovered by data miners.
With Ditto finally being included, plus a special Thanksgiving event taking place until November 30th, now might be a decent time to jump back into Pokemon Go.