From Bulbapedia, the community-driven Pokémon encyclopedia.

Capsule Monsters was Satoshi Tajiri's early design concept of Pokémon. According to the book Pikachu's Global Adventure, this early concept traces all the way back to Tajiri's childhood years, during which he had enjoyed bug collecting. As early as 1975, Tajiri published a manga series called Capsule Monsters, but sales of the manga failed.