Ditto is very elusive in Pokemon GO, and there is no guarantee that a player will catch one. But there are still Pokemon to look out for to acquire a Ditto in 2021.
Finding Ditto in Pokemon GO is difficult because it never appears as itself. The purple blob-like Pokemon can transform into any Pokemon it wants to, and it can breed in that form as well. Its abilities in Pokemon GO are no different, but the Pokemon it transforms into change depending on the update.
In 2021, there are several Pokemon that Ditto could transform into around the map. If it turns out to be one in the wild, Ditto will pop out and reveal itself to the player, effectively getting caught.
The list of potential Pokemon that could be Ditto is:
Any one of these listed Pokemon have the chance to appear that could actually be a Ditto when caught, but there are no guarantees when it happens.
How Ditto changes the forms it chooses in Pokemon GO
Ditto doesn't stay in the same Pokemon GO forms forever. There's a rotation of Pokemon that it goes through, usually with larger updates or additional Generations of Pokemon to the game. When there are significant changes, the Pokemon available as Ditto change.
One of the earliest Pokemon that could be a Ditto was Magikarp. Ditto wasn't originally a part of the game, added after the initial release. Magikarp became one of its first forms for a bit, until there were updates to Pokemon GO.
That update was to the game's shiny pool, which happens with major events or updates because not every shiny is available in Pokemon GO. As a new shiny is added to the game, it becomes unavailable for Ditto to use its form. Magikarp received a shiny version, and that's when Ditto had to move on.
The rotations for Ditto right now seem to be a way to avoid players catching a potentially shiny Pokemon and getting a Ditto instead. So, Ditto will only take form as a Pokemon without a shiny version, and that list will change.
The whole idea is to keep players from catching Pokemon that they needed, so much of Ditto's pool is reserved to Pokemon that are frequent or not rare.