Of the 88 Legendary and Mythical Pokemon that have been made available throughout the mainline series of games, 63 have been available in Pokemon GO for players to catch, battle, and show off. Of those 63, an even smaller number have been given forms in the popular mobile game for those who like to do some shiny hunting. It's important to note which Legendaries and Mythicals have received Pokemon GO shiny forms thus far so that players can look forward to which ones may be getting special shiny releases in the future.
Shiny hunting tends to be one of the most complicated tasks a trainer can do, with many only having a handful of shinies that they stumbled across on accident. Pokemon GO gives players the advantage of being able to see the Pokemon in the overworld, which can eliminate some of the guesswork of running through tall grass hoping to find a shiny that was required in past main-series titles. Adding raid battles to the mix can make catching things like shiny Moltres that much more complicated in Pokemon GO, and oftentimes only the luckiest or most determined players can acquire one. It's not impossible, though, so it's good to know which Legendary and Mythical shinies may be coming next.
The Missing Shinies in Pokemon GO
Galarian Bird Trio: The Legendary Bird Trio from the first generation have all received shinies in their main forms, but the Galarian variations still have no shinies that have been available to catch. The Galarian Bird Trio is notoriously hard to catch in Pokemon GO so shiny hunting them will likely be a nightmare whenever they arrive.
Armored Mewtwo: While shiny Mewtwo and shiny Mew have both been available in Pokemon GO, the variation of Armored Mewtwo has yet to get a shiny form, despite being available in multiple raid events in the past.
Shaymin: Shaymin, in both its Land and Sky forms, has been available through various Pokemon GO Fest research events, but the Sinnoh Mythical Pokemon doesn't come in a shiny form if the research is completed.
Victini, Keldeo, and Meloetta: The three mythical Pokemon from Unova are available in the mobile game through separate research events, but not in shiny forms.
Hoopa: This is currently the only Kalos Mythical currently in Pokemon GO since Diancie and Volcanion remain absent, and while Hoopa can be obtained in both its Confined and Unbound states, it won't be shiny in either case.
Cosmog and its Evos: Cosmog and its evolutions, Cosmoem, Solgaleo, and Lunala, became available to obtain through special research some time ago. The Cosmog players receive will always be the same color, though, which means no shinies of it or any of its evolutions are currently available.
Meltan and Melmetal: Meltan and Melmetal are currently the only two Pokemon exclusive to the world of Pokemon GO, which makes it a little strange that there's never been a way to catch a shiny version. That makes Meltan and Melmetal two of the only Pokemon to have no shinies available in any game.
Zacian and Zamazenta: The Legendary dog duo from Pokemon Sword and Shield have had two Raid Battle events in Pokemon GO, but neither of them have been for shiny forms.
Zarude: The rare Galar Mythical Pokemon was only available in Pokemon GO via a limited research event some time ago, and the players who finished the event did not get a shiny.
Regieleki and Regidrago: The newest Regis from Gen 8 had some pretty notorious Raid Battle events in Pokemon GO recently. Shinies weren't a possibility in either case, and it may be a while before they come back given the infamy around the original events.