Officially, the only Pokémon game available on the Apple Watch is Pokémon GO. That hasn't stopped one fan from bringing one of the classics to the platform, however. iOS software developer Gabriel O'Flaherty-Chan had been working on a Game Boy emulator for a few months before finally releasing it a few days ago. The emulator, named "Giovanni" after the head of the notorious Team Rocket, was designed to maintain a balance between framerate and performance, according to O'Flaherty-Chan. Using this emulator, O'Flaherty-Chan was able to port Pokémon Yellow to the Apple Watch.
Porting games to odd devices is nothing new within the community. For instance, in 2015, Doom (a benchmark for this sort of task) was ported to the Apple Watch by a couple of Facebook developers. O'Flaherty-Chan said that he used this as a basis for his program, updating it as necessary for the Apple Watch 2. This process didn't come without a few hangups, though. One was figuring out how to mirror the colors of the original game.
The bigger issue, though, is how to control the game. After all, if you port it over, certainly you'd want to play it, right? To solve this, O'Flaherty-Chan mapped the controls so that movement is controlled by panning the screen, tapping the screen activates the A button, and twisting the Digital Crown on the side of the device moves up and down through the menus. Everything else has a dedicated button on the screen. O'Flaherty-Chan wrote about this: "Touching the screen for movement isn't a great interaction, but being able to use the Crown worked out a lot better than originally anticipated. Scrolling through a list of options is basically what the Crown was made for, and if the framerate was even slightly higher, the interaction could almost be better than a hardware D-pad."
Giovanni is an open-source project, and O'Flaherty-Chan says he is open to working with other developers to simplify the emulator for other users. You can find out even more about this project over at the developer's website.