"Get ready for a beta gecko experience, baby!"
This beta version of Gex 3 was dumped by Hidden Palace only a few months ago and it reveals a lot. Dated a few months before release, there's content in this prototype that never made it to the final. The title screen and proper menus weren't even implemented yet!
While each level has at least a few differences, the stars of this beta version are the completely scrapped levels. Labelled under the "N64 Levels" menu, it appears that more exclusive levels were meant for the N64 version than just the different Mythology Network, which does exist in this prototype alongside the other one.
This video only scratches the surface of the levels this beta contains. If you want to see all of them, including the scrapped levels, check out this video from Hidden Palace, although they do skip a large portion of some of them through the debug free movement mode:
https://www.youtube.com/watch?v=GOtz-SdnJWM