From Bulbapedia, the community-driven Pokémon encyclopedia.
Capacia Island (Japanese: オオキナ島 Ōkina Island) is the ninth island Ash, Iris and Cilan visited during their travels through the Decolore Islands. It is the biggest island of the archipelago.
A shrine found on the island
In The Path That Leads to Goodbye!, the group encountered a Dunsparce here that Ash wanted to catch. It escaped due to Iris's Axew, however, creating a tension between Ash and Iris.
In Searching for a Wish!, the group continued their journey on this island and encountered a wild Jirachi awakening from its slumber. After this, they helped a girl named Gemma in her quest to ask Jirachi to grant her wish to make the land fertile again so her father would return home.
It also appeared in Capacia Island UFO!.
Pokémon seen on Capacia Island