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 Gems Collection
Developer Sonic Team conceived the compilation to introduce younger players to older "Sonic" games. One game they wished to include, "SegaSonic the Hedgehog", was excluded due to emulation difficulties. Sega released "Gems Collection" for the GameCube and PlayStation 2 in August 2005. Reviews were mixed or average; critics were divided over whether the package would satisfy players. They preferred "Sonic CD" and "Vectorman", but found "Sonic the Fighters" and "Sonic R" mediocre, and disliked the Game Gear games. Some were disappointed by the absence of the "Streets of Rage" games in the American version and other "Sonic" games like "Knuckles' Chaotix" and "Sonic the Hedgehog Pocket Adventure".