So we have this legendary trio that directly echoes the one from the previous gen that was made up of the fire bird Moltres, the electric bird Zapdos and the ice bird Articuno.
Entei has volcanic smoke on his back befit of his fire type theme.
Raikou has a thundercloud on his back befit of his electric type theme.
Suicune has an aurora borealis on her back which are phenomenons that happen in the polar arctic regions of the world, this is even manifested into the game via the ice-type move Aurora Beam. Suicune is said to be the embodiment of the north wind, said wind has historical and literary significance as a signal for cold weather. Suicune features a crystal shaped forehead crest suggesting the crystalline nature of ice. All these traits perfectly befit her ice type theme.