Sega Mouse
From Sega Retro
The Sega Mouse and Sega Mega Mouse are special mice designed to work with the Sega Mega Drive/Genesis and Sega Mega CD. Only a few games were designed with Mouse functionality in mind, including Art Alive, Wacky Worlds, and Eye of the Beholder. It was released as a response to Nintendo's own Mario Paint on the Super Nintendo, which came bundled with a mouse.
American version
The American version, called the Mega Mouse, had four buttons: , , , and START , just like a standard 3-button controller. It was packaged with a red-and-black mousepad.
Other regions
Other regions had the Sega Mouse, which had only two buttons but could be turned upside-down and the ball used as a trackball (the ball could be pressed to act as a third button). The European version came bundled with a blue mouse mat.