Mannequins are meant to be stationary objects that display armor inside houses. Supposedly, they are programmed as NPCs with a simple script designed to make them stand still whenever they are in the player's presence. However, when a building with a mannequin inside is loaded, if the mannequin's script isn't loaded fast enough they will walk around and act like normal NPCs for a short time, a glitch that causes occasional instances of mannequins spawning away from their pedestals, or just acting like they are alive before becoming stationary again.
Contributed by MehDeletingLater