9. Norimaro (Marvel Super Heroes Vs. Street Fighter)
This is a weird one, because for gamers in Japan, Marvel Super Heroes Vs. Street Fighter's Norimaro is in the game as a normal playable character. But for everyone else, he's absent completely, only discoverable by searching the game's ROM files.
Norimaro was created in a collaboration between developer Capcom and Japanese comedian Noritake Kinashi, and was intended to be a jokey yet very much playable character in all versions of the game. But for reasons unknown - although it's speculated to be something to do with licensing, a complaint from Marvel, or even simple cultural differences - he was removed from the overseas versions.
But with gamers being a nosy bunch, people started to do a bit of digging, and eventually uncovered some secret files (i.e. translated voice lines) that clearly showed the English-language version of Norimaro had been mostly completed, an indication that he wasn't given the chop until the very last minute.
These files didn't allow players to bring Norimaro into the game in a playable way, but they proved to be a cool look behind the curtain at what the overseas version of the character would've been like if he'd survived the chop.