Dinosaur Planet was planned to be Rare Software's swan song for the N64. It was availiable in a mostly playable form in E3 1999 or 2000 (the exact date is unknown), which showed off the huge levels and varied combat options. From what was shown of the game, it was an action-based 3D platformer that took a more realistic route with its graphics, and focused on two seperate characters, Saber and Crystal, in a world that appeared to be mostly inhabited by dinosaurs. Both characters were fully playable, appeared to have sidekicks or helpers, and specialized attacks.
In 2002, after the Gamecube was released and the game had to be ported over to the new console, it was changed to Star Fox Adventures, and suffered heavy changes in content. Saber was changed to Fox McCloud, Crystal was heavily demoted, entire scenes were removed, and the final boss fight was also cut, but information within the games files reveal some of what was planned. The game is also known to be low quality, using stock sound effects and an awkward, made-up language for some of the dialog, but from what is known of the N64 version, it had high quality graphics for the time, no made-up languages, and some fairly decent sound effects (it's possible many heard are placeholders).
No known prototype of Dinosaur Planet has ever shown up, and it's likely that the original version is completely lost by this stage.
The Unseen64 article, features piles of images and video: Retrieved March 23, 2015.