As mentioned already, Shiny Xerneas is available in Pokemon GO. The same methods used to obtain the standard Xerneas—winning raids or GO Battle League matches—apply to its shiny variant.
As the third legendary of the Kalos region, following Xerneas and Yveltal from Pokémon X and Y, Zygarde has often been overshadowed by other iconic legendaries like Mewtwo and Giratina.