Paramount Pictures in the summer of 2020 confirmed that the next Sonic the Hedgehog movie would open on April 8, 2022. Aside from what we gathered in the post credit scene from the first film, not much was known about the sequel, but that has since changed.
Recent photos from the set of Sonic the Hedgehog 2 show co-stars James Marsden and Tika Sumpter hard at work. In one particular shot, however, we see a crew member adjusting props for CGI characters Sonic, Tails and Knuckles. We of course knew that Tails would be joining Sonic in his next adventure, but the inclusion of Knuckles was a bit of a surprise.
Knuckles the Echidna was introduced in the 1994 Genesis game Sonic the Hedgehog 3 as an NPC. The game was developed alongside Sonic & Knuckles; the two were originally supposed to arrive as a single game, but time and budget constraints forced Sega to split the project into two.
Knuckles first became playable in Sonic & Knuckles, which arrived in late 1994 with “lock-on technology” that let players plug a second cartridge into the game. Attaching Sonic 3, for example, let players play Sonic 3 as Knuckles and Sonic & Knuckles levels as Tails.
To the best of my knowledge, this is the only game that ever used the lock-on technology.