Godzilla - Destroy All Monsters Melee - Platform: XBox

Cheat mode:
Hold L + B + R (in that order) at the main menu, then release B, R, L 
(in that order) to display the code entry screen. 

Boxing Ring level:
Enter 440499 as a code. A monster will roar to confirm correct code entry. 

Play as Destroyah:
Successfully complete adventure mode as Godzilla 2000. 

Play as Gigan:
Successfully complete adventure mode as Aguirus. 

Play as Godzilla 2000:
Enter 637522 as a code. A monster will roar to confirm correct code entry. 
Alternately, successfully complete adventure mode as Godzilla 90s version. 

Earn Godzilla 2000 
Simply beat the game on Easy with Godzilla 90's. You will see a little 
box when you beat the game that says you unlocked a monster