From Bulbapedia, the community-driven Pokémon encyclopedia.
Monsu Island (Japanese: モンス島 Mons Island) is an anime-only location in Hoenn. It is located between Lilycove City and Mossdeep City.
It made its only appearance in Gaining Groudon, in which Ash and Co. unknowingly boarded a ship that belonged to Team Magma. Monsu Island is the location of the bases of both Team Magma and Team Aqua.
Pokémon seen at Monsu Island