In this South Park episode – http://en.wikipedia.org/wiki/The_Death_Camp_of_Tolerance – Lemmywinks, the class gerbil, is inserted into Mr. Slave’s rectum and then goes through . . . Well, it seems to be some kind of mystical quest to find his way out the other end. He is guided by mysterious spirits, apparently ghosts of other animals who died there. Was this based on a video game, or something?
Perhaps you missed tihs part of the Wiki page?
I hadn’t seen that old Hobbit cartoon for years, and I’ll tell you–watching it again after having watched Lemmiwinks recently was a real hoot. 
(And watching Lemmiwinks again after watching the Hobbit is equally amusing.)
If you’re asking why he was inserted, as memory serves Mr. Garrison was intentionally being outrageous in class because he wanted to get fired and sue the school board for $20 million claiming it was because he was gay, so when sexually graphic conversation and the like fail to get him fired he shoved a gerbil up Mr. Slave’s ass in front of the class. Still didn’t work.
The Quest itself also reminded me a little bit of the old TV movie The Point.