Answer: Deer
Clark and the family were on their way to get a Christmas tree. While they drove to their destination, Clark and Ellen sang Christmas songs. A truck pulled up behind the family vehicle and was riding their bumper. The truck driver sped up and passed Clark. This made Clark upset and he pulled up along side the truck. Clark said, "Look, kids, a deer," and as Ellen and the kids looked, Clark gave the truck driver an obscene gesture.
Chevy Chase and Beverly D'Angelo reprised the roles from the first two movies. The children were played by entirely different actors which is the norm for the "Vacation" films.