According to Sonic in Archie's Sonic the Hedgehog #188, he's been fighting Scratch, Grounder, and Coconuts since he was at least 8. We don't seem to see these fights at the time, as Sonic is older in early Archie, but he's about 16 at the time of this statement.
In IDW's Sonic the Hedgehog #45, Garfield can be spotted with his best friend, Pooky.
In Archie's Sonic X #34, Eggman is revealed to have an attraction to Ella, the housemaid to the Thorndyke family.
In the official Sonic the Hedgehog 25th anniversary comic featuring Eggman Nega, he teleports away from Blaze using the same kind of portal used by himself and various bosses in the story mode of Mario & Sonic at the Olympic Winter Games for Nintendo DS.
In Volume 2, Chapter 18 of the Sonic the Hedgehog manga "Dash and Spin: Hyper Speed Sonic", Dr. Eggman creates a film called "The Hyper Speed Hero's Hidden Shame 9/11", which consists of Sonic taking a piss.
Amy Rose seems to be the only character in the main Sonic the Hedgehog cast who can handle a casual punch from Knuckles.
Scott Pilgrim's high school band "Sonic & Knuckles" is a very subtle reference to the 1994 Sega Genesis game Sonic & Knuckles.
Sonic the Hedgehog does not know what hedgehogs look like.
Whisper the Wolf's design began with an in-game custom character in Sonic Forces. From there, she would gradually evolve into her final look.
Tails is very discontent with the fact that Eggman has settled on making Tails Doll rather than a Metal Tails. Moreover, this suggests that he finds Metal Sonic and Knuckles to be particularly cool.
When robots or lackeys fail, Eggman uses his built-in command grab
In 2015, it was revealed that Ian Flynn, Tracey Yardley!, and Ben Bates of Archie Sonic the Hedgehog fame had pitched a line of Super Mario comics to Nintendo. It was ultimately rejected. Allegedly, Mario was also intended to cross over with Sonic and Mega Man in comic form.
Knuckles the Echidna's father dies of pancreatic cancer in the Archie Comics.