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.
Ristar
Back in late 1994, Sega was originally pitching "Ristar" to be the successor of Sonic the Hedgehog. However, the game never received a ton of exposure or sales, mainly due to being released just three months prior to the Sega Saturn, Sega's successor to the Genesis, overshadowing it. In a 1994 "Electronic Gaming Monthly" interview, Sega marketing staff Lisa Best and Terry Tang claimed "Ristar" was not designed by the same "Sonic" programming team, although much of the game's staff would later go on to create "Nights Into Dreams" for the Saturn, the next game to be officially credited to Sonic Team. This, along with "Ristar's" inclusion in a number of "Sonic" themed compilations and re-releases in subsequent year, would lead game journalists to retroactively label the game as being developed by Sonic Team.