There is a Game Boy game published in 1991 called Faceball 2000. It's a first person shooter that supports up to 16 players with a homebrew hardware connector. This was even before Nintendo's own 4-player adapter was available for any games. The connector was not Nintendo-endorsed so it could not be shipped with the game but the code is still there in the ROM. Disclosure: I was a programmer at the small company responsible but not the genius himself. You can read a short interview with him here.[1]
[1] https://web.archive.org/web/20100509163809/http://fb2k.retro...