Doubutsu Banchou [Animal Leader]

Developer: Saru Brunei / Marigul Management, Inc.
Players: 1

"When animals were still human the world was filled with the untamed. Wildness dwelled in everything and life sprung up everywhere. However, animals spent days in tedium and the wild was gradually forgotten. The wild started to crumble and become scattered about. In this animalistic action game, Doubutsu Banchou , survival of the fittest is the law of the land. By using wild, untamed instincts to fight and feed off of other animals, you must crawl your way to the top of the animal kingdom and bring the wild back to civilization."

Originally planned for 64DD this was moved to N64, then put on hold and revived on Gamecube as 'Cubivore'.
As far as I can tell the NGC version is exactly the same as the N64 one. Basically you move your creature around the areas attacking, killing and eating parts of other creatures to increase the number of forms you change into, at the end of each stage you woo mates and have a child who you then play with in the next level.

Movie one: Battle sequence