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 &amp; Knuckles
For the series' twentieth anniversary in 2011, Sega released "Sonic Generations", a game that remade aspects of various past games from the franchise. A remake of the Sky Sanctuary stage was made for the Xbox 360, PlayStation 3, and PC versions of the game, while the Mushroom Hill level was remade for the Nintendo 3DS version of the game. "Sonic & Sega All-Stars Racing" features a track inspired by the Death Egg as downloadable content along with the Metal Sonic character. The sequel, "Sonic & All-Stars Racing Transformed", featured the same race course, and an additional new one inspired by the Sky Sanctuary stage. Re-imagined versions of the Flying Battery and Lava Reef stages, along with the Blue Sphere bonus game, appear in the 2017 game "Sonic Mania".