This information was found mainly from Sonic Retro.
Some of you might have remembered a while back I posted a few blogs updating you guys about an unknown game called Sonic Extreme. No, not Sonic X-Treme, the famous Sonic game that went into development hell. This game was an early pitch demo in around 2004 or 2005, which ended up becoming the game we all know as Sonic Riders.
The game is available to play at the Portland Retro Gaming Expo.
With all this known, Sonic Extreme has been confirmed as, not a fan game, but a game in the early development stages that evolved into Sonic Riders. Sonic Retro now has a page for the game, and gives some information as well.
This means it is now okay to create a page for the game, now knowing it has been confirmed.