Xerneas has been released in Pokémon GO, but its Shiny form isn't out yet. However, Shiny Xerneas is available to see due to Niantic's coding for the game. Let's take a look below at Shiny Xerneas as it will appear in Pokémon GO.
As it is, Xerneas has two forms, but they don't operate the same way as capital "F" Formes as with Deoxys and Giratina. Instead, Xerneas has two modes: Neutral Mode and Active move. Active Mode is pictured to the left, where Xerneas has glowing rainbow and yellow antlers. On its Shiny form, you can see that its black and blue body is replaced by white and light blue, but that its yellow and rainbow antlers remain the same, as do the neon yellow markings on its legs. In its Neutral Mode, Xerneas in its non-Shiny form (pictured left) is black and blue, with light blue antlers and leg markings. In its Shiny form, Neutral Mode Xerneas has dark blue antlers, a white and light blue body, and dark blue markings on its legs.
These modes don't have different stats, but rather act as a visual signal as to how Xerneas is being used in Pokémon GO. Neutral Mode Xerneas shows up in your Pokémon storage, but if you click on it, it switches to Active Mode. In battle, Xerneas is always in Active Mode as well. However, if you make Xerneas your Buddy, it will appear in Neutral Mode there as it's not fighting but rather spending some quality time with you. To photograph Neutral Mode Xerneas, make it your Buddy. To photograph Active Mode Xerneas, you must select Xerneas from your Pokémon storage.
Now, it's likely a long time until we will have Shiny Xerneas in the game, but it's always interesting to see what is coming. Personally, I love the way that Pokémon GO adapted these Modes to the game, allowing us to photograph and interact with Xerneas in both modes through different actions. This is partly why I think it's right to push back on the idea that a Pokémon with a bad moveset is "useless," as we have seen some influencers state. Pokémon GO is about more than just battle. Even with Shiny Xerneas unreleased, there are fun ways to interact with and even create photographic art with this unique Legendary.