Sonic 2 Megamix is likely a fan-made ROM hack with extensive modifications and additions to Sonic the Hedgehog 2. The term "Megamix" implies a significant number of changes, potentially including new levels, characters, game mechanics or updated visuals.  The project demonstrates the expansive creativity of the Sonic ROM hacking community and isn't developed by Sega.

Sonic 2 Megamix

Sonic 2 Mania Run Sprite