So it turns out that Sonic appeared in another game aside from Rad Mobile before he’d star in his own game. Youtube personality Guru Larry found out that his first appearance was actually in the incredibly obscure Amiga game The Adventures of Quik & Silva. The game was developed by Factor 5 under the pseudonym New Bits on the Ram, and included lots of copyrighted characters as enemies. One of those enemies being Sonic, which the developers seemingly based on pre-release screenshots.
The game was first released as a cover disc in the UK with issue #5 of Amiga Fun on May 10th, 1991. Almost a month before Sonic the Hedgehog would be released on the Genesis. Be sure to watch Guru Larry’s video about this if you want to know the full details.
Update: Twitter user @UltimateDSman pointed out that TheTetrahedronVG actually made a video about this. And so he did, back in 2011. That being said, Guru Larry does provide a lot more info, making it still worth watching if you already knew about this.