In Yokohama, Japan the Pokémon GO Pikachu Outbreak event has commenced and part of that included being able to find and capture a Shiny Pikachu.
— りょうちゅ (@KARINGA_first) August 9, 2017
Also, some players are reporting a Shiny Zapdos is appearing too although dataminers believe that could be a mistake with Pokémon GO Hub reporting
If you take a look at any of the Zapdos caught in your Pokedex, or at your nearby Gym, you’ll see that the Zapdos model currently available in game is actually a shiny Zapdos model, not the regular one. Unfortunately, this doesn’t imply shiny Zapdos is available. It seems to be a problem completely on their end, as the models are mistakenly swapped – the game is downloading the shiny, instead of the normal, variant.
There is no word on if or when the Shiny pocket monsters will be available outside of the Pokémon GO Pikachu Outbreak event.