Shinies that you have never caught may be currently in your Pokédex. With this unique feature in Pokémon GO, trainers are now able to see Shiny Pokémon that they don't have… including some that have yet to be released in the game. Here's how to do it.
Do you have a Shiny Deoxys? If so, go to the Hoenn region in your Pokédex. Scroll to the bottom and click on Deoxys. Shiny Deoxys has only ever been released in its Normal Forme, but you can click and see the unreleased Shiny forms of Attack Deoxys, Speed Deoxys, and Defense Deoxys in your Dex without ever catching them. What is going on?
This is a feature that essentially exploits the idea of "registering" a Pokémon. For instance, if you catch as Shiny Alolan Sandshrew and go to trade for a Shiny Kanto Sandshrew, the trade cost will be dramatically discounted because your Pokédex sees you as having a Shiny Sandshrew registered. Largely, it doesn't discriminate between forms. Using this exploit, you can see, for example, Shiny Kanto Vulpix if you have Shiny Alolan Vulpix. You can see Shiny Origin Forme Giratina if you have Shiny Altered Forme Giratina. You can see these Pokémon you've never caught as if they're yours, right there in the Dex.
This may be more of a glitch than a feature, as it was never announced in Pokémon GO but it was also never fixed… and it's been active for a long time. The most interesting aspect of this is seeing Pokémon that don't yet exist in their Shiny forms, such as Shiny Galarian Meowth pictured above. Another example is Farfetch'd, which you can see in its (admittedly unimpressive) Shiny Galarian form if you've been lucky enough to register the original Kanto Shiny. Same with the stunning Shiny Galarian Zigzagoon.
One thing that's fun about this feature is when new forms roll out. Galarian Ponyta is already in the code of the game and since Shiny Kanto Ponyta has been released for quite a while, players will be able to view the Shiny Galarian in all of its pastel glory as soon as the standard Galarian shows up in the Dex.
There's no real use to this aspect of Pokémon GO, but it gives players an exciting glimpse at the future of the game with just a few clicks.