Sega Neptune
From Sega Retro
Sega Neptune was a two-in-one Sega Mega Drive/Genesis and Sega 32X console that Sega planned to make.
Sega had admitted how expensive and problematic the 32X was. Sega decided to make a combined version of the Megadrive/Genesis and 32X, which they felt was a better idea. However, by the time a prototype came out, the Sega Saturn was ready for release. Sega felt that gamers would now not be interested in the Sega Neptune, if it had been released, so the project was scrapped.
Had the Neptune been released, it would have been available for $400.
In 2001, gaming magazine EGM announced that prototype copies of the Neptune had been found at a warehouse and were up for sale to the public (via the now defunct Seganeptune.com website). This was an April Fool's joke, but generated a lot of buzz amongst the Sega community.