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 the Hedgehog (8-bit video game)
Porting the original game to the 8-bit hardware was impossible, so Ancient built their "Sonic" from scratch. The team decided to make their version completely different from its Genesis counterpart. According to Koshiro, the game had three phases of development. In the first phase, Ancient developed the game with the Genesis version in mind. The second and third phases were largely Ancient's own ideas, with Sega supervising their work. Koshiro thought reinventing "Sonic" for 8-bit hardware was challenging, as he did not work at Sega and had never developed a game before. However, Sega had faith in him because of his relationship to the company. Koshiro composed the soundtrack and sought to retain the feel of the Genesis version. He converted Masato Nakamura's 16-bit "Sonic" score to the 8-bit programmable sound generator to start, but ended up using only three of those tracks; the remainder of the music is Koshiro's work.