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.
History of Sega
To compete with Nintendo, Sega was more open to new types of games than its rival, but still tightly controlled the approval process for third-party games and charged high prices for cartridge manufacturing. Technicians from American third-party video game publisher Electronic Arts (EA) reverse engineered the Genesis in 1989, following nearly one year of negotiations with Sega in which EA requested a more liberal licensing agreement than was standard in the industry before releasing its games for the system. As a result, EA signed what founder Trip Hawkins described as "a very unusual and much more enlightened license agreement" with Sega in June 1990: "Among other things, we had the right to make as many titles as we wanted. We could approve our own titles ... the royalty rates were a lot more reasonable. We also had more direct control over manufacturing." The first Genesis version of EA's "John Madden Football" arrived before the end of 1990, and became what EA creative officer Bing Gordon called a "killer app" for the system.