So how you can catch Ditto in Pokemon GO?
Ditto takes the form of a few specific Pokemons, that’s why the best way to catch it, is to look and catch those Pokemons.
Ditto transforms into this Pokemons in Pokemon GO:
Zubat is presumably no longer a Ditto disguise candidate as it’s now available in its shiny variant, because of the Team GO Rocket’s event in Pokemon GO. (Ditto can’t transform itself to a potential shiny Pokemon)