To enter these cheats go to the "SECRETS" option on the title screen or in the pause menu during a game then enter these combinations.

All Keys:
Lizard, Dragonfly, Bull, Bear, Wolf, Eagle.

All Weapons:
Owl, Bear, Owl, Insect, Hawk, Owl.

Big Hands/Feet:
Lizard, Lizard, Dragonfly, Horse, Lizard, Coyote.

Big Heads:
Cougar, Wolf, Snake, Rabbit, Lizard, Coyote.

Clean Screen. Displays Off:
Rabbit, Owl, Lizard, Elk, Salmon, Rabbit.

Credits:
Elk, Elk, Elk, Elk, Elk, Elk.

Gourad (funny textures):
Lizard, Salmon, Insect, Salmon, Wolf, Dragonfly.

Headless Enemies:
Lizard, Elk, Eagle, Owl, Salmon, Horse.

INVINCIBILITY!!!!!:
Raven (purple), Salmon, Eagle, Bear, Lizard, Rabbit.

Mannequin (Strange stuff in deathmatch mode):
Snake, Bull, Snake, Frog, Bear, Elk.

Menu Madness:
Rabbit, Owl, Horse, Insect, Bear, Bear.

Pen & Ink Mode:
Cougar, Horse, Elk, Fish, Lion, Hawk (red).

Stick Mode:
Horse, Eagle, Snake, Cougar, Insect, Salmon

Gasping Mode:
Dragon Fly, Bull, Rabbit, Salmon, Eagle, Raven

Tiny Enemies:
Frog, Frog, Salmon, Insect, Wolf, Cougar.

Unlimited Ammo:
Salmon, Elk, Bull, Snake, Eagle, Salmon.

Warp Codes (you can play boss of any chapter or start from beginning of that chapter.)
Level 1: Frog, Elk, Horse, Dragonfly, Wolf, Rabbit.

2: Owl, Owl, Horse, Elk, Elk, Elk.

3: Owl, Rabbit, Bear, Insect, Frog, Cougar.

4: Bear, Horse, Raven, Eagle, Horse, Coyote.

5: Bear, Dragonfly, Horse, Bear, Frog, Elk.

Hints

Thanks to Revolution readers Da Dark Platypus, Mackenzie Chami and Chris Havens!