Concept art of how Sonic runs around loops.
Marble Zone Artwork
Concept art showing how Sonic takes to water.
Concept art showing how Sonic goes through tubes.
One of the ides for Sonic was a bulldog.
Eggman was going to be the good guy? Yep, he was originally designed as the protagonist of the game. The fact that he was a fat, mustached man, seemed to fit better as a villain though. (As a knock at Mario.)
Another idea for Sonic was an American wolf. (Thank god that didn't work.)
Does the alligator look familiar?
Some of the first sketches for Sonic as a hedgehog.