ps3punk7890 has found another interesting beta leftover in Jet Force Gemini (relased in 1999): a racing track from Diddy Kong Racing (relased in 1997)! This level is the Greenwood Village and it looks almost the same as the one in DKR, apart from some missing trees and a differend sky. This hidden track could have a simple explanation: Jet Force Gemini was build on a enhanced version of the DKR 3D engine, so this track was left behind when the code was ported and probably it was even used by the RARE beta testers as a test-level for the racing mini-games that are in JFG. Below you can see the leftover map in JFG (first video until 0:37) and the original one in DKR (second video):
Update: it seems that the DKR track is really in JFG! As Andi has made us to notice: “This ISN´T any beta-stuff! I have this game and I´ve unlocked this Diddy Kong Racing-Track absolutly normal! You must collect all Tribals and other stuff and you will unlock this easter egg! Nothing BETA! ;-)”