Kyogre helps summon the rain that frees Onix in Pokémon Pokopia.
The sequence begins when Ditto finds an Onix trapped in a cave. Tangrowth suggests that rain could soften the rock, but the region has been dry for years. Slowpoke’s yawns produce a short shower, and that rainfall awakens Kyogre from its resting place at the bottom of the sea.
After Ditto befriends Kyogre, the Legendary Pokémon provides the proper rainfall needed to make the cave rock breakable. This event also raises Ditto’s Trainer rank enough to open the gates to Bleak Beach and Rocky Ridges.
Groudon is the most obvious wrong answer because it is Kyogre’s famous land-based counterpart, while Rayquaza is associated with the sky. Suicune is also linked with water in the wider Pokémon series, but Kyogre is the Pokémon used for this specific Pokopia story event.