Rayquaza was available in Raid Battles from February 9th until March 16th, 2018.
Rayquaza was available again from March 15th to March 18th, 2019 during Special Raid Weekend. However, shiny Rayquaza is not released.
Rayquaza is available again from July 31st to September 2nd, 2019.
Shiny form of Rayquaza was released on July 31st, 2019.
In the Shiny Rayquaza promotional image, there are three traditional Chinese characters in calligraphical style. The large one behind Shiny Rayquaza means "black", while the two small ones on the side mean "descent" or "arrival".