The Kanto Tour event in Pokémon Go highlighted the first 150 Pokémon introduced in the series, celebrating the first batch of games in the series. The event was a way to celebrate Pokémon's 25th anniversary, giving players the chance to catch all of these Pokémon and release all brilliant versions of these Pokémon. That includes Ditto, but Ditto was a special case for the event where he was only available for the Special Research reward. So how does capturing a brilliant Ditto work now?
Previously, after the Kanto Tour event, it was not possible to capture a brilliant Ditto in the wild. Even if a shiny Pokémon turned out to be a Ditto, that Ditto wouldn't be shiny. It would be the standard version. After September 2021, things will work out a little differently.
Now there is a chance for players to catch a brilliant Ditto in the wild. Players have a high chance of catching a Bright Ditto when disguised as standard wild-caught Pokémon. However, a Shiny Pokémon that can transform into Ditto will not transform into one. Only the standard versions will become Ditto. Similar to the other common Pokémon in the mobile game, there is a one in 450 chance that a Pokémon will be lucid.
The odds are low and the odds are against everyone, but catching more Pokémon and hunting the specific Pokémon Ditto can become important. Idem's transformations also rotate quite often.