Knuckles in Sonic 2 © 1994 Sega.
Not actually a separate game, but a special combination of Sonic and Knuckles and Sonic 2. On the original hardware, this is played by plugging Sonic 2 into the Sonic and Knuckles cartridge.
Despite the appearances, this is not actually a combination of the two games the way that Sonic 3 is with Sonic and Knuckles. Instead, this is a dedicated copy (a prototype) of Sonic 2 edited to include Knuckles embedded in S&K, and this is only accessed if Sonic 2 if plugged into the game at power on.