Do Rarity's clothes horses have any plastic in them? I was under the impression that they were made of cloth and stuffed. But hey, the idea of Autons in the Carousel Boutique is an entertaining one. I can just imagine Rarity's reaction.
Plastic mannequins are for displaying finished clothes. Cloth mannequins are for designing and creating clothing. Lot easier to stick pins in 'em. So maybe Rarity has both. Cloth ones for creating, and plastic ones for displaying her creations.