Raikou appears after Peakychu powers up in Pokémon Pokopia.
Peakychu is a peculiar pale Pikachu encountered in Bleak Beach. Although Peakychu has lost the ability to generate electricity, it can still absorb electrical power. Ditto repairs enough electricity sources to activate a charging station for Peakychu.
Once Peakychu has charged, its illumination helps light the area. That light is strong enough to bring Raikou into the story, and Raikou drives away the clouds covering Bleak Beach.
Raikou belongs to the Legendary beast trio from the Pokémon series, alongside Entei and Suicune. The other two trio members are plausible alternatives, but the Pokopia sequence specifically features Raikou. Its arrival also helps wake Mosslax from its exceptionally long sleep.