The Flintstones Movie (partially found Sega Channel exclusive build of 2D platform game; 1995)
Title screen for the Mega Drive/Genesis prototype of The Flintstones.
Status: Found (prototype)
Lost (final release)
Date found: 31 Dec '19 (prototype)
Found by: drx (prototype)
The Flintstones was a platformer developed by Ocean in 1994 for the SNES, with a Game Boy version developed by Twilight. This game was based on the 1994 live-action movie of the same name, which was based on the cartoon created by William Hanna and Joseph Barbera in 1960. What is lesser-known, however, was that a Sega Genesis/Mega Drive port of the game was created and developed by Foley Hi-Tech in 1995, but was released digitally through Sega's paid Sega Channel service, a service that allowed subscribers to download games compatible with the service for a limited time, as opposed to being released physically.
As this port is mostly identical to the SNES version, the gameplay involves Fred Flintstone traversing through 7 levels attempting to rescue Pebbles and Bamm-Bamm.
While the SNES version can be easily found, the final build of this Sega port has yet to have resurfaced online. Prototypes are known to physically exist, but haven't resurfaced online either until very recently. The only known screenshots of this version to exist come from an issue of Computer & Video Games from April 1995. Since the Sega Channel automatically deleted downloaded games after turning the console off, the final release of this port may never resurface unless its developers happen to still own copies of the game and manage to get the licensing rights to rerelease it.
On December 31, 2019, as part of a release event, Hidden Palace user drx had dumped an early prototype for the game sourced from an EPROM cartridge. This prototype version of the game only has two levels and no sound. 
- Garfield: The Lost Levels, a pack of levels from Garfield: Caught in the Act exclusive to the Sega Channel. These levels have also been lost.
- Wikipedia page on The Flintstones. Retrieved 11 Aug '19
- Sega-16 Forum page from 2009 on this game. Retrieved 11 Aug '19
- Hidden Palace page on the game, and provides a download link to the prototype. Retrieved 04 Jan '20
- SegaRetro page mentioning the existence of prototypes. Retrieved 11 Aug '19
- Hidden Palace page announcing the release of the prototype. Retrieved 4 Jan '20