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.
Sega Technical Institute
Sega Technical Institute (STI) was an American development division of Sega. Founded by Atari veteran Mark Cerny in 1991, the studio sought to combine elite Japanese developers, including Sonic Team programmer Yuji Naka and his team, with new American talent. STI developed a number of "Sonic the Hedgehog" games, as well as other games for the Sega Genesis, before it was closed at the end of 1996.