Sonic The Hedgehog CD (cancelled Sega CD port of Sonic The Hedgehog; 1992)
Sonic the Hedgehog CD was a planned port of Sonic the Hedgehog to the Sega CD intended as the definitive way of playing Sonic the Hedgehog. It was showcased at the Consumer Electronic Show (or CES) to show the power of the Sega CD. It was also teased in issue #11 of Mega Play. A possible reason for its cancellation is that it occurred in favor of marketing Sonic CD as the Main Sonic title of the medium.
Only a handful of screenshots and some footage is available online, so it remains lost. However, a fan remake of Sonic the Hedgehog to the CD does exist, made by Sonic retro user HCKTROX.