Sonic the Hedgehog
The Sega Genesis had "add-on" systems that incorporated "Sonic" games. "Sonic CD", released for the Sega CD, was a 2D platform game released in 1993. The game introduced the characters Amy Rose and Metal Sonic and featured levels that differed depending upon whether Sonic was in the past, present, or future time frames. "Knuckles' Chaotix", a spin-off released in 1995 for the Sega 32X, featured Knuckles and a new group named Chaotix fighting against Dr. Robotnik. The game featured a two-player cooperative system in which the on-screen characters were connected by magic rings. There was also "Sonic Eraser", a puzzle game released exclusively on Sega Game Toshokan in 1991.
Sonic Chaos
Sonic Chaos is a platform game developed by Aspect and published by Sega. The game was released for the Master System in October 1993, and for the Game Gear the following month. The Game Gear version was re-released in 2003 as a hidden game in "Sonic Adventure DX" for the GameCube and Windows and as an included game in "Sonic Mega Collection Plus" on the PlayStation 2, Xbox and PC. In 2009, the Master System version was released on the Virtual Console.