From Bulbapedia, the community-driven Pokémon encyclopedia.
Donto Island (Japanese: ドント島 Donto Island) is an anime-exclusive location in Date Expectations, located between Mirage Island and Pacifidlog Town. It is home to a large number of Donphan. Even though it seems unpopulated by humans, it does have a small harbor on one side of the island, which has a ferry service to Pacifidlog Town.
Ash, Brock, May, and Max were on their way to Pacifidlog Town for May's final Contest when the engine of the boat they were on broke and they were unable to continue. The captain, Carlos told them that it might take a few hours to fix and that they should be able to take the boat to Pacifidlog Town on the other side of Donto Island, were they were close to at the moment.
Ash and his friends entered the island and quickly found out the island was highly populated by Donphan. They were attacked by a Donphan that used Rollout but stopped it with the help of Mudkip, and Corphish. They became friends with the Donphan that attacked them and found that it was attracted to a female Shiny Donphan. At first, the female Donphan wasn't interested in the Donphan Ash and his friends helped, but after a confrontation with Team Rocket they managed to get the two in love with each other.
They finally reached the other side of the island but found out that they were too late and the ferry to Pacifidlog Town already left. Just in time, however, Carlos returned with his repaired boat and he managed to get Ash and his friends to Pacifidlog Town.