Castform is a unique Pokémon in that it has four different forms depending on the weather conditions, but only one Shiny form. Until now. Niantic has updated the coding for Pokémon GO, and dataminers have uncovered newly added sprites for the Shiny versions of Castform's Shiny weather variants. These were previously seen in Pokémon HOME, but it seems as if this addition is unique in that Pokémon GO will feature these Shinies before they're available in the main series games.
Confirmed by the Poke Miners, Niantic added these new forms today, September 17th. There has not been word of any event surrounding these Pokémon, and this addition to the code does not yet mean that these three new Shiny versions of Castform are available. Pokémon such as Mew, Spearow, Ditto, and others have Shinies in the code but, without Niantic switching them on, remain unavailable.
That said, this Castform update is intriguing because of the length of time that this Pokémon, in the main series games, was only available as a Shiny in its normal form. This new Shiny release includes Sunny Castform, which sees its head turn a blazing red while its body takes on a pink accent. Rainy Castform's body turns fully blue, while the pattern around its eyes glows green in its Shiny form. Finally, Snowy Castform, perhaps the rarest variation for most players, sees its normally translucent body turn a frosty white as the blue of its head deepens and the pattern around its eyes switches to yellow.
There is precedent for Pokémon debuting in Pokémon GO before the main series games, but that is limited to the release of Meltan only. It is likely that there will be a Weather-themed event surrounding the release of these new Shiny Castform variants. As Niantic updates, we will follow closely.