Miles "Tails" Prower makes a highly-anticipated appearance.
Just as Tails was introduced to Sonic the Hedgehog canon for the character's 1992 video game sequel, the third and fourth SEGA Genesis entries featured the first appearances of Knuckles the Echidna. A striking red figure, very similar to Sonic in appearance, Knuckles was another popular addition to the ever-expanding cast of characters on South Island, and remains visible on the franchise's merchandise range even into 2020. Although Knuckles' journey begins with the echidna as an antagonist, his animosity towards Sonic and Tails is later revealed to have been engineered by Robotnik himself, and the echidna teams up with his fellow animals to take the scientist down.
Although Knuckles doesn't physically appear in the Sonic the Hedgehog movie, the opening sequence contains clues that could lead to his introduction in Sonic the Hedgehog 2, if a sequel is made, and the crimson character could help explain some lingering mysteries. When Sonic the Hedgehog begins, a young Sonic is told by his protector, Longclaw, that he must keep his super-speed powers hidden, and when Baby Sonic fails to heed this warning, he and his adopted parent are attacked by a strange tribe who kill Longclaw and force Sonic to flee to Earth through the golden rings. Although not immediately clear, the most interesting part of this scene is that the tribe are all echidnas.
The connection that this tribe and Knuckles belong to the same species is unlikely to be a coincidence, especially since there's so much about Sonic's new origin the movie leaves ambiguous. The audience never finds out why the tribe were chasing Sonic, or how Longclaw knew Sonic's powers would bring unwanted attention. When Tails appears in the post-credits sequence, it's clear that Miles is looking for Sonic, strongly suggesting he might know something about the hedgehog's origins and powers, which would naturally tie back into the tribe from Sonic the Hedgehog's establishing flashback scenes. If this proved to be the case, it would make sense if Knuckles were the leader of the strange echidna tribe, and Tails has arrived on Earth to warn Sonic about him.
This scenario would fit with the original games' initial depiction of Knuckles as a villain, and add another layer of video game continuity to Sonic the Hedgehog's alien opening. It would also allow any potential Sonic the Hedgehog 2 to explore how Sonic's powers came to be, and why he was forced into running away for most of his life. While Knuckles could serve as a great antagonist for a sequel, he'd no doubt eventually team up with Sonic & Tails against a bigger villain, and this would be a massive video game movie moment.
