Kyogre summons the lasting rain that helps free Onix in Pokémon Pokopia.
The story first links rain to Slowpoke, whose yawns can cause rainfall. However, Slowpoke’s rain is too brief to solve the problem. That event awakens Kyogre, the Legendary Pokémon associated with the sea and powerful rain.
After Ditto befriends Kyogre, Kyogre summons the proper rainfall needed to soften the rock trapping Onix in a cave. Freeing Onix advances the story and raises Ditto’s Trainer rank enough to open routes to Bleak Beach and Rocky Ridges.
Slowpoke is therefore the main alternative players may remember, but it is not the final answer. The question asks which Pokémon produces the lasting rain that completes the rescue. Kyogre’s role also connects Pokopia’s gentle town-building story with a recognizable Legendary Pokémon from the wider series.