From Bulbapedia, the community-driven Pokémon encyclopedia.
The deserted island is an anime-exclusive island in Unova that Ash and his friends visited in Expedition to Onix Island!.
Jervis took Ash and his friends to that island since Dawn suggested to the group that they take a trip to the island to take a break from training for the Pokémon World Tournament Junior Cup. On the island was a Shiny Onix that the group wanted to go see. They searched for it after lunch. The group went in a cave and encountered some Onix that separated Ash and Cilan from Dawn and Iris.
As the group was searching for each other, Oshawott and Piplup drove away the Onix that Ash and Cilan encountered. Meloetta also took part in the battle. Since Piplup was separated from Dawn, Dawn and Iris had to run away when one of them was chasing the two. As the Onix lost sight of them, the two continued their way back to the beach. Along the way, Iris saw a Magnemite and Dawn saw a Foongus. It didn't go well for either of them as both of them were attacked by the two Pokémon.
After the group found each other, they had to deal with several more Onix. As Dawn reunited with Piplup, she took part in battling the Onix. As Ash, his friends, and Meloetta were no match for the Onix, the Shiny Onix appeared and ordered the normal-colored Onix away from Ash and his friends. After the group got to see the Shiny Onix, Jervis took them back to Cynthia's villa.
Pokémon seen on the island