Every Pokemon game in the series has introduced at least some Legendary critters that are directly tied to the story in some way or represent elemental forces that are integral to the region's lore, climate, or overall phenomenons. An example is how closely tied to Hoenn's map and narrative Groudon, Kyogre, and Rayquaza are, or how relevant Zacian and Zamazenta are to the overarching Darkest Day plot, including the Eternatus ending. All Legendary Pokemon can be shiny, even if not all of them have already been available in their color-swap version.