The Sega Neptune was a prototype console created by Sega. It was designed to be a Sega Mega Drive and Sega 32X all-in-one console, due to the expensive nature of both the Mega Drive and 32X combined, and was an effort by Sega to spike new interest and sales into 32X games.
Although, it never got passed the prototype stage. It was dropped due to new hype being generated for the Sega Saturn, with the console selling moderately well, Sega decided to stop work on the Neptune and decided to concentrate fully on their Saturn.
There are very few working prototypes in the world, and if one is ever advertised on eBay, it would go for an extremely high price, much like the Dreamcast ZIP Drive.
It was black in color, and had two cartridge slots, one for the Mega Drive games, and one for 32X games.