The original Japanese name of the Pokémon creature series was Pocket Monsters. The Japanese games were titled Pocket Monsters Red and Pocket Monsters Green when they launched in 1996.
The shorter international name Pokémon comes from the contracted form of “Pocket Monsters.” It was used to create a distinctive trademark that could work across languages and regions while retaining the idea of collecting small creatures.
The franchise’s Japanese name is also written as Poketto Monsutā. International releases changed the game titles to Pokémon Red and Blue in many markets, while the anime and merchandise adopted the Pokémon name. “Pocket Monsters” therefore describes the original branding, not a separate game series or a later generation.