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 2
For Sonic's 20th anniversary, Sega released "Sonic Generations", which remade aspects of various past games from the franchise. The PlayStation 3, Xbox 360, and PC versions contained a remade "Chemical Plant" level. It also contained a remake of the final boss fight, the Death Egg Robot, as the Classic Era boss of the game. Separately, the Nintendo 3DS version of the game contained a remake of the "Casino Night" level. A "Casino Night" themed pinball minigame was made available for download as a pre-order bonus for the console versions at GameStop. Remade versions of Chemical Plant and Oil Ocean also appear in the 2017 game "Sonic Mania".