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.
Knuckles' Chaotix
Development of "Knuckles' Chaotix" began in early 1994 as an engine test for the Sega Genesis dubbed "Sonic Crackers". Although Sonic Team is sometimes credited for developing the game, it was developed by another internal Sega development team including young members of the staff who had worked on "Sonic CD" (1993). Developed as an idea for the next "Sonic" game, the prototype featured Sonic and Tails joined together by an elastic band of energy. Development eventually moved to the more powerful 32X add-on for the Genesis. Sonic and Tails were replaced by a group of characters led by Knuckles the Echidna, but the core tethering gameplay was retained. The project was titled "Knuckles' Ringstar", and later "Knuckles' Chaotix". The "Sonic Crackers" prototype ROM image was later leaked online and can be played with emulators.