What name was Pokémon originally developed under before it became Pocket Monsters?

Pokémon was originally developed under the name “Capsule Monsters.”

Satoshi Tajiri's early concept drew on the idea of creatures being stored in capsules and released for play or battle. The project was commonly shortened to “Capumon” during development, but the name Capsule Monsters could not be trademarked.

The concept was subsequently renamed Pocket Monsters, which became the Japanese title Poketto Monsutā. The English brand name Pokémon is a romanized contraction of “Pocket Monsters,” with the accent mark helping distinguish the name in international markets.

The name history is sometimes mixed up with the first game's official Japanese title. Capsule Monsters was an early development name, not the title used when the first games launched. Pokémon Red and Green were released in Japan in 1996 under the Pocket Monsters branding, after years of development and several changes to the project's name and scope.