Sonic Saturn (lost build of cancelled Sega Saturn prototype of platformer; mid-1990s)
Alongside the production of Sonic X-treme, Sega Technical Institute was working on another game only known as "Sonic Saturn". Rather than use sprites for sonic, though, it was going to use a 3D model for Sonic. This game was meant to have a more realistic rendition of Sonic's world than X-treme. It was never actually announced publicly that it was in production, but didn't get very far before being canceled.
The bonus rounds of Sonic Saturn were going to be a game of pool. After the cancellation of Sonic Saturn the pool game was being considered to be included as a bonus round in the Saturn version of Sonic 3D. Due to time constraints, this too was abandoned in favor of a 3D version of Sonic 2's special stage.
The concept for the game was Sonic would stand behind a ball, use his Spin Dash and aim his direction then smash into it. The camera would then switch to an overhead view and show the resulting action as he smashes into the other balls. The goal was to knock all the balls into some portals within a certain time limit. It was possible to push the balls but it is slower than spinning into them. Later stages would have enemies walking around, damaging Sonic if he touched them. Other arenas were also being developed, such as a South-Western courtyard, among others.