From Bulbapedia, the community-driven Pokémon encyclopedia.

During the planning and development stages of Pokémon FireRed and LeafGreen, several parts were going to be a part of the games. Some of them were dropped or changed, but some of those dropped concepts of the beta version can still be found in the game's programming.

Locations

The islands in the Sevii Islands region would likely be named Sevii Isles. In addition, several more locations in the Sevii Islands have been found in the programming code, including Sevii Isles 6-9 and 22-24. It is very notable that Sevii Isle 8 and 9 have fully coded map data, implying that they were scrapped very late in the game's production.

Beta versions of Pokémon games
Generation I | Generation II | Generation III (RS | FRLG) | Generation IV