Dragon's Lair was a hugely popular laserdisc arcade game in the early 80's. It was ported to everything, NES, SNES, Gameboy Color, CD-i, even the Coleco Adam received a port on a cassette tape. The Genesis version never saw the light of day but was very far along in development before it was cancelled. Since the Genesis didn't have the power to handle the FMV that the game was known for the game was converted to a side scrolling platform game with levels that were inspired from scenes in the arcade game.
The prototype was near complete, playable all the way through and the only thing missing is an animation when the player crouches and throws a weapon.
It's also way better than the NES version, but then again so is just about anything else.