Sega Mouse
From Sega Retro
The Sega Mouse and Sega Mega Mouse are a special mouse designed to work with the Sega Mega Drive/Genesis. Only a few games were designed with Mouse functionability in mind, Art Alive, Wacky Worlds the Sega CD game Eye of the Beholder, and a few others. The unit was released as a response to Nintendo's own Mario Paint, which came bundled with a mouse designed to be used with the Super Nintendo.
American Version
This version came with a total of four (4) buttons: 'A', 'B', 'C', and 'Start', just like the standard 3-button Genesis/Megadrive control pad. It was also packaged with a red-and-black mouse mat.
Other Regions
This version has a mere two buttons but it could be turned upside-down and the ball used as a trackball. If the ball is pressed in it acts as a button press. The European version came bundled with a blue mousemat.