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 Generations
Sonic Generations is a 2011 platform video game developed by Sonic Team and published by Sega for the PlayStation 3, Xbox 360, Nintendo 3DS, and Microsoft Windows. An installment in the "Sonic the Hedgehog" series, the game—which was produced in commemoration of its twentieth anniversary—follows Sonic and his sidekick Tails as they form an alliance with their past selves to stop an evil entity from erasing all time. It features two gameplay styles: "Classic", which plays from a side-scrolling perspective like that of the original Sega Genesis "Sonic" games, and "Modern", 3D levels similar to those in "Sonic Unleashed" and "Sonic Colors".