KT15 found unused sprites in Green Hill Zone. These two animations are unused, however, another animation is used, but it is used in Jungle Zone, not Green Hill. This sprite is only found in the Master System version of the game.
This Bat sprite appears in Green Hill Zone but isn't used. It was probably meant to be in the caves of Act 2. In the 16-bit version this is a normal enemy in Marble Zone, a level not found in this version. This sprite is only found in the Game Gear version of the game.
One of magazines shots features the screenshot from possible, earlier build of the original Master System game with the hud spelling "RING" instead of "RINGS".