Knuckles the Echidna in Sonic the Hedgehog was a planned add-on for Sonic the Hedgehog which allowed you to play as Knuckles by attaching the cartridge to Sonic & Knuckles. The game was only previewed once in volume 44 of SEGA Megazone.
The game was later replaced by Blue Sphere in the final release of Sonic & Knuckles.