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 Mega Collection
"Sonic Mega Collection" features twelve to fourteen complete games in one retail unit. All games are ports of their original versions on the Sega Genesis. High-resolution instruction manual scans are included for each game. Also included is an "Extras" section featuring over 100 scans of "Sonic the Hedgehog" comic covers, illustrations of "Sonic" characters throughout the franchise's history, and a handful of videos promoting other "Sonic" games, including the beginning and end sequences to "Sonic CD". In addition to the seven default games, three unlockable games are included that recreated the "Sonic & Knuckles" "lock-on technology" by allowing players access to new gameplay features and special stages by opening the hatch of the "Sonic & Knuckles" Sega Genesis cartridge and inserting another compatible cartridge, in this case each of the first three games in the "Sonic the Hedgehog" series. Also, four games published by Sega but not in the "Sonic" series could also be unlocked (two in the North American version).