Sonic Unreal Worlds v1 is a fan-made game or ROM hack, a likely earlier version of Sonic Unreal World v1.4. The 'v1' designation points to the first public release of the game.  Expect some differences compared to the later version. The original Sonic games were created by Sega, while this is a community-created work. It demonstrates the potential for fan-made games to introduce original concepts while building upon existing game designs.

Sonic Unreal Worlds v1

Sonic Unlimited v3.0