In Pokémon Pokopia, Kyogre summons the proper rain that allows Onix to be freed.
The sequence begins when Ditto discovers an Onix trapped inside a cave that is too difficult to break through. The characters learn that rain could soften the rock, but the area has not experienced rain for years. Professor Tangrowth therefore sends Ditto to find Slowpoke, whose yawns are said to summon rain.
Slowpoke does produce a brief rainfall, but it is not enough to solve the problem. That short shower awakens Kyogre, the Legendary Pokémon associated with expanding the oceans and controlling rain in Pokémon lore. Kyogre then helps summon a proper, sustained rain, softening the obstruction so Onix can be released.
Groudon is the key mix-up because it is another major Legendary Pokémon from the Hoenn region and is associated with land and drought. In this Pokopia story, the rain-bringing Legendary is Kyogre.