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
Super Attacks for Bonus Monsters
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.
All VMS 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.
Giant Man Character
If you start a normal game using Godzilla, then USA Godzilla, then Mini Godzilla, a giant man can be used in the game.