This includes two pieces that depict the characters introduced in the post-credits scene: Amy Rose and Metal Sonic. The art of Amy closely resembles her introduction in the final movie.

Metal Sonic and Amy Rose’s introductions in the Sonic movies are undoubtedly the best ... Additionally, Tacit Sign Studio released the concept art for Sonic the Hedgehog 3, which was reposted ...

There is even leaked Sonic 3 concept art which reveals that the movie either almost included Robotnik using time travel or featured a parallel universe. Another still shows Metal Sonic standing ...