Sonic Encore (early development version) is a fan-made game for the Sega Genesis; the "early development version" indicates that this is a prototype or early iteration of the full game. Details regarding the developers are usually minimal for such projects. The gameplay would be similar to classic Sonic games, but with elements potentially altered or incomplete.  The title suggests that more complete versions may exist or are planned.

Sonic Encore (early development version)

Sonic Delta v0.75