The Scaredy Ghost is designed to be scared by the spooky trick or treaters.
The arms are open to look like a normal ghost. When the scary trick or treater gets close, the ghost covers it's eyes because it is scared.
It uses a PIR sensor to detect the trick or treater, fed into an Arduino nano. There are two servos, one in each arm to open and close the arms. When the closed up ghost detects no motion, it will carefully open one arm to "look" if the trick or treater is still there. If they are, it will quickly close the arm again, otherwise it will continue opening the one arm, and then the other.
I 3D printed the frame from my own design. The frame is draped with some gauze like cloth. The cloth only affects the PIR slightly. Just used a sharpie for the eyes and mouth. This is an early movie of the servos and arms.
There was a scaredy ghost on hackaday a few years ago, but it was made of flat cardboard.