From Bulbapedia, the community-driven Pokémon encyclopedia.
Cave Island (Japanese: 洞窟島 Cave Island) is the thirteenth island that Ash and his friends visited during their travels through the Decolore Islands.
Prior to the group's arrival, Clair flew on her Dragonite to the island and somehow got separated with her. By the time the group arrived on the island, Clair was still searching for Dragonite. Alexa temporarily left Ash and his friends to find something to film. Ash then reunited with Clair and Iris and Cilan met her for the first time. Ash and Iris volunteered to help Clair search for her Dragonite using Charizard and Dragonite respectively. When Iris's Dragonite found Clair's, he became interested in meeting her but she got into a fight with him when he trampled on some flowers she was treasuring.
On the other side of the island is a cave that was home to a Shiny Druddigon that Clair heard about. Team Rocket also heard about it and used one of their devices to lure it out of the cave. However, this also caused other Pokémon to leave the cave due to the loud noise the device made. With the Druddigon out, it went in a fit of rage and despite Team Rocket's efforts to catch it, Druddigon sent them blasting off. Clair then tried to calm it down and was successful. Druddigon ultimately decided to join Clair's team. Alexa eventually came back to the group and Clair showed her the Shiny Druddigon that she caught.
Pokémon seen on Cave island