LogFAQs > #978621158

There are no reoccuring enemies in the franchise aside from Eggman, there have been several non Eggman affiliated antagonists tho starting from classic era

The battle bird armada who ended up in conflict with Tails
Wendy Witchcart and the Witchcarters who ended up in conflict with Tails
Fang the Hunter who ended up in conflict with Sonic in his debut game Triple Trouble even through he wasn't affiliated with Eggman and Knuckles who at the time thought Sonic was an enemy

That's it for classic era for the modern era non Eggman enemies that kept coming back didn't exist until the Deadly Six, through so far they've only returned as antagonists in the IDW comics

the closest thing to a reoccurring antagonist outside of the Deadly Six in modern sonic is Eggman Nega he turned 3 times before Sega got rid of him, however his debut and return the first two times he was working with Eggman, the other two times he's actively antagonizing both the protagonists and Dr.Eggman

There's just no beating Eggman as far as the fanbase is concerned same with Sega mainly because all their other bad guys aren't nearly as popular or liked as Eggman or they don't have the will or desire or demand for a non eggman enemy as a main stay at least in the games, IDW comics have several reoccurring antagonists Mimic the infiltrator, Clutch the Possum, Rough and Tumble the skunk brothers, Surge the ternec and Kit the finnox, it used to have another enemy named Starline a duck billed platypus but he was killed off two years ago because they had run out of ideas for him.