There are some leftover files from development.
In any version of THUG, the Skateshop from THPS4, a Testlevel, and an Artist viewer level (SP) can be loaded with some modifications. The files for these levels are in the Pre folder.
On the PS2 THUG disc, the Levels directory contains some files for Team HQ, College, London, a Gun testlevel (WWW), two Motox (MTX MotoTrax) testlevels, a level named Australia, and CarTest, but they lack the collision and qb files to be loaded in game to be played, and don't have the texture files to be loaded on the original system.