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
"Mega Collection" was developed by Sonic Team, the studio behind most games in the "Sonic" series. According to director Yojiro Ogawa, Sonic Team's goal with the compilation was to introduce children to older games in the series and showcase what made "Sonic" successful. Sonic Team chose to focus on including all the Genesis "Sonic" games. Rather than porting them from the Genesis, they collaborated with VR-1 Japan to develop an emulator to run the games' ROM images. Series co-creator Yuji Naka, who served as producer, said the team intended to include the 1993 Sega CD game "Sonic CD", but storage constraints prevented this. Sonic Team also considered including BlueSky Software's "Vectorman" (1995), and "Sonic R" (1997) and "Sonic Shuffle" (2000) were expected to appear in "Mega Collection" at one point. "CD", "Vectorman", and "R" later appeared in "Sonic Gems Collection" (2005), a successor to "Mega Collection" focusing on rare "Sonic" games. Sonic Team had trouble gathering materials for the compilation because it had been a decade since they made the games and "Sega's not that good about keeping history." For instance, Naka wanted to include the original prototype of the first game but Sonic Team could not find its ROM image.