From Bulbapedia, the community-driven Pokémon encyclopedia.
Cerosi Town (Japanese: セルドスタウン Cerdos Town) is a town located on Izabe Island in the Hoenn region. It is never seen on screen, but mentioned in Let it Snow, Let it Snow, Let it Snorunt! and Do I Hear a Ralts?. Ash, Brock, May and Max were traveling here because it has ferry services to Sootopolis City.