"Capsule Monsters" is the Earliest Pokemon Game concept, designed by the Nintendo's Employee Satoshi Tajiri in 1990.
According to Pikachu's Global Adventure, he was inspired by his childhood experiences. He loved too much catch and collecting bugs.
For unknown reasons, Tajiri changed the name of the project from "Capsule Monsters" to "Pocket Monsters" ( Legal and copyright issues, maybe ? )
According to the sketches, some creatures could bought in shops. The player didn't had to fight in order to catch pokemon, instead he had to check a Charisma stat that had to be raised to entice wild monsters to join them.
The Player also didn't controlled the Pokemon during the fights, according to this sketch