It's a common misconception that Nintendo owns the Pokémon franchise, to the point where it caused stock problems when Pokémon Go became a hit, as investors weren't clued in on the existence of The Pokémon Company. No one could have anticipated that Pokémon Red & Green would form the basis for the biggest media franchise on the planet, least of all its developers.
Pokémon Red & Green were developed by Game Freak and published by Nintendo. The development of these games was fraught with issues, to the point where a last-minute delay led to the games being pushed out of their copyright year. The bugs and the technical issues did little to stop Pokémon Red & Green from becoming a huge hit, leading to an anime series, a trading card game, and various manga that are still running to this day. Pokémon Red & Green are still the best-selling games ever in Japan, though Animal Crossing: New Horizons is a close second.
The vast majority of the Pokémon games have appeared on Nintendo platforms. There are a few exceptions, such as the Pokémon games on PC, mobile phones, and the Sega Pico, but Pokémon is closely associated with Nintendo. The abundance of Pokémon in the Super Smash Bros. series is a testament to this, as only the Mario franchise has more representation in terms of fighters. The truth regarding Pokémon ownership is a little more complicated, and Nintendo is just part of the story.
The Three Entities Of The Pokémon Company
Pokémon is owned by The Pokémon Company, which is composed of three separate organizations: Creatures, Game Freak, and Nintendo. Creatures was formerly known as Ape, which was the company best-known for the EarthBound/Mother series. Creatures' involvement in the Pokémon franchise involves the Pokémon Trading Card Game and handling the massive amounts of merchandise that the series produces. The company is also involved in the development of the Pokémon video games, especially the 3D entries in the series.
Nintendo owns a 32% stake in The Pokémon Company, while the other two companies own the remaining amount. 4Kids Entertainment (the original dubbers/localizers of the Pokémon anime) briefly owned part of the company, but they sold their stake in 2005. There used to be separate entities that represented the Pokémon franchise in different territories (like Pokémon UK and Pokémon USA) but they have all been folded into The Pokémon Company, which is now a global entity.
The role of The Pokémon Company is to continue building the franchise as a multimedia empire, which is why players have started to see live-action Pokémon movies. The profits from Pokémon Go are split between Niantic, Nintendo, and The Pokémon Company, with all companies seeing huge returns from the game. Despite the split ownership of the Pokémon franchise, it's highly unlikely that Pokémon will ever go multi-platform, as Nintendo would need to be bought out for such a deal to go through. Pokémon Sword & Shield sold twenty million copies on Nintendo Switch, so the Pokémon franchise isn't going anywhere anytime soon.