Codes
At the Secrets screen, enter any of the following:

Elk, Elk, Elk, Elk, Elk, Elk - Credits

Cougar, Horse, Elk, Salmon, Cougar, Hawk - Pen and Ink Mode

Raven, Salmon, Eagle, Bear, Lizard, Rabbit - Invincibility

Owl, Bear, Owl, Insect, Hawk, Owl - All Weapons

Salmon, Elk, Bull, Snake, Eagle, Salmon - Unlimited Ammo

Frog, Elk, Horse, Dragonfly, Wolf, Rabbit - Warp Level 1

Owl, Owl, Horse, Elk, Elk, Elk - Warp Level 2

Owl, Rabbit, Bear, Insect, Frog, Cougar - Warp Level 3

Bear, Horse, Raven Eagle, Horse, Coyote - Warp Level 4

Bear, Dragonfly, Horse, Bear, Frog, Elk - Warp Level 5

Lizard, Dragonfly, Bull, Bear, Wolf, Eagle - All Keys

Lizard, Elk, Eagle, Owl, Salmon, Horse - Headless

Lizard, Salmon, Insect, Salmon, Wolf, Dragonfly - Gouraud

More modes
At the Secrets screen, enter:

Snake, Bull, Snake, Frog, Bear, Elk - Mannequin mode

Horse, Eagle, Snake, Cougar, Insect, Salmon - Stick Man mode

Frog, Frog, Salmon, Insect, Wolf, Cougar - Tiny mode