This one may look like a stretch, but there are many hardware-exclusive games out there, and it would be the same as the Arcade platform, which isn't, by itself, a concise hardware platform, but a concept.
The only handheld electronic game option on IGDB now is the Game & Watch, but there are many games, even in renowned franchises, for things like the Tiger Electronics and other smaller brands.
The one I got to know recently was the Nelsonic Game Watch series (don't confuse with Game & Watch), which had some games licensed by Nintendo itself, like The Legend of Zelda Game Watch and Super Mario Bros. Game Watch.

Wikipedia on this platform: https://en.m.wikipedia.org/wiki/Handheld_electronic_game