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's Ultimate Genesis Collection
Sonic's Ultimate Genesis Collection (Sega Mega Drive Ultimate Collection in PAL regions) is a compilation of video games developed by Backbone Entertainment and published by Sega for PlayStation 3 and Xbox 360. The compilation features 48 Sega games (49 counting two versions of "Altered Beast") which were previously released for the Sega Genesis (including most of the "Sonic the Hedgehog" titles released for the system), arcades and the Master System. It is the sequel to the "Sega Genesis Collection" released previously for the PlayStation 2 and PlayStation Portable, however it is similar to that compilation because it contains 16 (in NTSC regions) more games (including unlockable extras).