You can now get a shiny Keldeo in Pokémon Go, if you bought the "Pony Tales" Masterwork Research ticket for $7.99.
As usual with Masterwork Research, this research was only purchaseable for a limited time (and it's not available anymore), though once you bought and started the research, you'd have it unlocked permanently — which is a good thing, because it'll take a while to complete this research.
Below we list out the known steps for the "Pony Tales" Masterwork Research and talk a bit about whether the ticket is worth buying.
Update (Jan. 20): We've updated this guide with step five.
We don't have a full list of the rewards as of writing this (and we won't by the deadline to purchase the ticket), but the rewards for these types of tickets do usually make up the coin value of the ticket cost, making it "worth it" in that sense. However, the main thing to consider here is if you want a shiny Keldeo.
Keldeo is a mythical Pokémon, and it has only had a few event distributions since its reveal. You should have access to a regular, non-shiny Keldeo as part of the "Tales of Transformation" seasonal Special Research, but if you specifically want a shiny one, this is one of the only ways to get one.
The only other legitimate method to get a shiny Keldeo is by completing the Pokédex for Pokémon Sword and Shield in Pokémon Home. To do this, you need to catch every Pokémon in Sword/Shield and then register those Pokémon in Pokémon Home. (You functionally need to have a living Pokédex in the aforementioned games, and then transfer everything to Home.)
If you don't think you ever want to do the above challenge, and you really want a shiny pony, well... this is your other option.
You can see the steps below for the research.
Rewards: 25 Cobalion Candy, 25 Terrakion Candy, and 25 Virizion Candy
Rewards: 10 Snivy Candy XL, 10 Tepig Candy XL, 10 Oshawott Candy XL
Rewards: 1 Incense, 30 Ultra Balls, 1 Rare Candy XL
Rewards: 1 Lure Module, 30 Ultra Balls, 1 Rare Candy XL
Rewards: 1 Max Mushroom, 30 Ultra Balls, 1 Rare Candy XL
Rewards: Shiny Keldeo T-Shirt, Keldeo encounter
Rewards: 6,470 XP, 6,470 Stardust

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