Play as original Godzilla:
Hold L + R and press any button at the monster selection screen. Alternatively,
successfully complete the game once. 

Play as Mech Godzilla:
Hold L + R and press any two buttons simultaneously at the monster selection screen. 

Play as Beta Godzilla:
Hold L + R and press any three buttons simultaneously at the monster selection
screen. 

Play as American Godzilla:
Hold L + R and press any button four times at the monster selection screen.
Alternatively, successfully complete the game as the original Godzilla. 

Play as Minilla:
Successfully complete the game as American Godzilla. 

Play as Dr. Serizawa:
Successfully complete the game as Minilla. 

All VMU monsters:
Enter the "VS Battle" menu and select the bottom option on that screen. On the next
screen, hold X + Y and press Down + L + R to unlock up to forty-four monsters. Note:
Only less than ten monsters will appear in the VMU.