1) Sonic’s character’s age is fifteen. That would explain his boundless energy, fearlessness and arrogance.
2) When the folks at Sega were deciding on a new mascot, Sonic was up against an armadillo character, a dog, and Theodore Roosevelt wearing pajamas (who later became Dr. Robotnik), and a rabbit.
3) Sonic’s original codename was “Mr. Needlemouse”
4) Sonic’s shoes were inspired by a pair of Michael Jackson’s boots.
5) Sonic’s personality was based on President Bill Clinton’s “Get it done” attitude.
6) Sonic dies if he falls in water because programmer Yuji Naka mistakenly thought that hedgehogs could not swim.
7) The original Music for Sonic the Hedgehog was composed by a member of the Japanese pop group “Dreams Come True.” Sega then sponsored a tour of theirs, putting Sonic’s image on the side of their bus and playing clips from the game before their shows.
8) Sonic initially had fangs
9) Sonic also initially had a human girlfriend named “Madonna”
10) In the American Sonic cartoons, Sonic was voiced by actor Jaleel White. Yes, Urkel.
Above facts from: http://www.holytaco.com/ten-little-known-facts-about-sonic-the-hedgehog/
And my own Number 11:
11) Super Sonic was inspired by the Super Saiyans in Dragonball Z:
Note the similarities?
First image from: A lost Fortunecity page
Second image from: http://s979.photobucket.com/profile/SonikkuHedgehog, By the artist SonikkuHedgehog
Third image from: http://delgatron-x.deviantart.com/art/Awesome-Sonic-198024882?moodonly=86, by the artist Delgatron-X