Evil alien prank
These guys are evil. Funny as hell, but evil. All I did was make my sister think she was adopted, but then again we didn't have nightvision camcorders and YouTube back in the early 80s.
And even though it's just a mannequin, mannequins are creepy, right up there with big dolls with those big staring eyes. I remember having childhood nightmares about that kind of shit, including headless mannequins turning people into headless mannequins. And when I woke up the first thing I saw was my dad's suits hanging in the (doorless) closet with those big old wooden hangers designed to help them keep their shape, so it looked like a closet full of headless mannequins. I was probably two at the time.