The goldfish writing the word "Kill" on the fish bowl
The alien creature that emerges from the space ship at the beginning.
When the pet store owner opens the "Employees Only" door revealing a portal to the doppelganger Cartman's world.
the Indian Burial Ground Pet Store is a reference to this movie
When the boys ask Cartman why he is at the bus stop, he responds sarcastically by asking them if they were expecting Maury Povich.
Stan tells New Kid/Douchebag when going back to his room "Nice goldfish, I had one like that once, then it started killing people so we had to get rid of it." Cartman's evil goatee can be worn as an accessory.
Mrs. McCormrick coming into the Mash's house drunk after hearing about Kenny's death and what he used to do comes from this film spoofing a scene in which Mrs. Daigle does the same thing after her son was murdered.
When the kids agree to carve the squash and enter it into the pumpkin carving contest Stan says "Yeah. You know, I never thought it was such a bad, little squash. It just needs some tender, loving care."
Evil parallel universe, counterparts wear goatees
Cartman tricks the boys into sending the Cartman doppelganger back to his home dimension by saying "Time is running out. You'll have to destroy us both."