Game Freak developed Pokémon Red and Green for their original Japanese release.
Game Freak began as a video-game fanzine created by Satoshi Tajiri and later became a development studio co-founded by Tajiri and Ken Sugimori. Its team designed the game systems, creatures, map, story, and battles that formed the foundation of Pokémon.
Nintendo published the games and supplied the Game Boy platform, so Nintendo is often mistakenly given the developer credit. The companies had different roles: Game Freak made the games, while Nintendo handled publishing and hardware-related support.
Creatures also became a major Pokémon partner, especially through character design, licensing, and the trading card game. Niantic arrived much later as the developer of Pokémon Go, not the original Game Boy titles.