Ubisoft's Assassin's Creed series is one of the most recognizable in the games biz today, but you might be surprised to learn that it actually started out life as a Prince of Persia spin-off, called Prince of Persia: Assassins. Early concept footage for the game that might have been was discovered by Superannuation in a work reel belonging to a former Ubisoft sound designer (via Eurogamer).
You don't really get a sense here of of how Prince of Persia fits into what you're seeing, which is probably why Assassin's Creed grew into its own thing. There's a lot that's familiar for fans of Ubi's history-trotting franchise, however. Skip to 1:30 for that footage. There's some other unreleased stuff here too, including a look at a never-released 3D Indiana Jones game.