You can access these hidden monsters by completing certain monsters all the way to Tokyo's three stages.

• Godzilla-1st. -- Beat the game as Godzilla
• Godzilla-USA -- Beat the game as Godzilla-1st
• Minilla -- Beat the game as Godzilla-USA
• Dr. Serizawa -- Beat the game as Minilla

Godzilla-USA does a really fast run. Use left or right directional pad to turn. Easy to control.

Minilla shoots smoke rings. Hard to control.

Dr. Serizawa activates the Oxygen Destroyer which completely demolishes any structure, tank, jet or helicopter it strikes. Damages any Super X.

You can unlock all monsters in the vms up to 44 of them by going to the VS Battle, selecting the third choice (bottom one). Once in this screen, hold x and y buttons, then press down on the d-pad and press the trigger buttons at the same time. This individually unlocks each monster up to 44. However, when you go to the Stadium Battle, the vms only tends to hold the first 8 to 10 monsters. This must be repeated every time the game is turned off.