My final design differed from my initial idea in several ways. The drawing shows a simple lever to raise he lid of the hat. In the real version, I used a four-bar linkage. By experimenting with the lengths of the linkages, you can actually get a motion that moves both up and out. This give the monster a more natural motion and really suggests that it is trying to scare you.
My original design also shows that the lid of the hat pulls the monster up. In the final design it's the other way around: as the monster rises, the linkage he is attached to lifts the lid of the hat out of the way.
Finally, I had planned to make my own monster out of wood. I simply ran out of time the year I built the original hat, and used a rubber finger puppet instead. It turned out to be a great choice. The yellow puppet is highly visible in low light and his upraised rubber arms shake around when he appears. Very comical.
To learn more about making animated displays for various holidays check out Animatronics: Guide to Holiday Displays.