NFL Blitz (lost arcade prototype; 1997)
NFL Blitz was a sports game by Midway Games that was released in 1997 for arcade systems, Nintendo 64, PlayStation 1, Windows, and the Game Boy Color. It's had many incarnations since its release across multiple other systems, but the original NFL Blitz prototype was reported to have originally been far more violent and "arcady" than its final product.
The prototype was actually tested in some arcades and featured more violent elements such as blood splatters, a "neck-breaker" move, and more violent animations, one of which was a longer dog pile animation with more screaming from the victim. Despite the high praise and success of the prototype tests, representatives of the NFL were very upset at the high level of violence, demanding that they tone it down. They eventually did, and the game was released in a less violent form, leaving the original prototype unused and lost.
No dumps of the prototype have been found online and no known physical "copies" of the prototypes have been found.