Originally introduced during the Tour: Kanto event, a Shiny Ditto is a bit trickier to capture than a regular Ditto in the wild. Similar to the regular Ditto, you’ll be able to be rewarded with one during the September Research Breakthrough and the Kanto Special Research quests this month, so you’ll have more than one opportunity to bag this tricky Pokémon.
Unfortunately, if you’re looking to capture one on your own, it’s a bit trickier. The Shiny Ditto will be disguised as all of the same Pokémon as the regular Ditto this month, though it’s not as easy as attempting to capture a shiny version of those Pokémon.
All Shiny Gastly, Drowzee, Remoraid, Teddiursa, Gulpin, Numel, Dwebble, and Foongus Pokémon will not be a Shiny Ditto in disguise, so as not to ruin an opportunity for trainers to capture the shiny version of any of those Pokémon.
Instead, you’ll have to continue to catch regular versions of those Pokémon, and it’s possible one of them will be a Shiny Ditto. Again, there is no way to determine if the Pokémon is actually a Ditto until you capture it.