Enter the following as your Profile name:

BUBBA             All riders
GENERALLEE        All ATVs
REDROOSTER        Champs
DOUBLEBARREL      All challenges
ROADKILL          All tracks
FIDDLERSELBOW     All tricks
GINGHAM           Max Stats

Bonus ATVs:

Beat the Academy Training and Trick Training to unlock the Dimension and Viper

ATVs. Win more than eight races in Arcade mode to unlock the ATC ATV.

Thanks to Revolution reader Dan!