When on the character selection screen, highlight the wrestler you want to play as. Instead of choosing with X, hit the Start button. I think all the wrestlers have a second outfit, but I'm not sure. Here are a couple:
Hulk Hogan:
Press the Start button to get his old yellow "Hulkamania" duds.
Sting:
Press the Start button to get him with blonde hair and different clothes (his old style).
Alternate wrestlers
At the character selection screen, highlight Hulk Hogan, Sting, or Masahiro Chino and press Start instead of X to play their old versions.
Rally Cross 2 Demo
To gain access to the Rally Cross 2 Demo, pause the game during play and press L1, L2, R1, R2, Up, Up and resume gameplay.
Random wrestler
At the character selection screen, press Circle.
The Giant and Jaguar
To get the Giant or Jaguar, you must choose one wrestler (cruiserweight to get Jaguar, heavyweight to get Giant) and beat the six League Challenges. This will open up a new league: Super Jr./Hvyw. Beat that with the same wrestler and you will earn the Giant/Jaguar. Go through this twice to earn all 8 secret wreslers!