For complete animatronics kits, have a look at https://www.bottango.com/pages/kits They are not cheap, but very educational.

You could buy a HalloWing from Adafruit and build something around it: https://www.adafruit.com/product/3900

Many Halloween effects can be done with a simple wiper motor: https://www.youtube.com/channel/UCeBxc0fj-UUayo01ei0B7wQ

Also very popular for instructions: https://www.youtube.com/channel/UCKKBTwYyVYxkpa0_9pqQ13A Steve Koci, the creator, has sadly passed away, but his book "The Ultimate Guide To Do It Yourself Animatronics" might be worth a look.