Niantic have released the famous Golden Magikarp and the shiny Red Gyarados into the wild.
Trainers across the globe have reported sightings of golden Magikarp in the wild. These Shiny Magikarp are extremely rare, so pay close attention to the Magikarp you encounter while you’re out exploring the world to make sure you don’t miss the opportunity to catch one!
The post states that they will be "extremely rare" to find. What are the chances the Magikarp is a shiny?
The shiny rate appears to have been updated since shiny Magikarp was first released. The observed shiny rate, from The Silph Road is 1 in 450 for random wild encounters. With that data, the observed Magikarp shiny rate was 1 in 360 (95% CI 310 - 440), but testing if there was a difference in rate between all the shiny species collectively was not statistically significant. So it's still assumed to be 1 in 450.
According to The Silph Road, the spawning chance of shiny magikarp is approximately 1/235
Moreover, it seems that the chaining is a thing in pokemon GO too. Chaining is a technique used in the main games where encountering the same pokemon repeatedly increased the chance of a shiny pokemon encounter. Make sure you only enter in combat with magikarp for a while and to capture them.
