Pokemon GO’s new event helps increase the chances of running into water-type Pokemon. While that is awesome in itself, there was a stealth release included in that update that made shiny hunting a possibility in the real world.
The only known Pokemon that have shiny forms are Magikarp and Gyarados, which can be found in the wild though it’s not as easy as it sounds. While the idea of getting a red Gyarados and showing it off at all the local gyms sounds exciting, it will actually be fairly hard to find one. This is because Gyarados are already rare to begin with, so finding a shiny Gyarados will be a bit harder.
Your best bet will be to look for a shiny Magikarp since they are easier to find in the wild, especially with the current water event going on. Go to your local hunting grounds, find a Pokestop and make sure to keep lures on it at all times to increase your odds. Sadly, the Pokemon does not appear shiny on the map itself so you’ll have to go after every Magikarp you see. Once you do find one, then feel free to evolve it into a shiny Gyarados, or wait until you find another one so you can keep both forms.