MLP are made from the same vinyl plastic as any other "doll line"....vinyl is a type of plastic made from ethylene and chlorine, its processed and they combine to form Polyvinyl Chloride (PVC) resin or as its more popularly known, vinyl.
Its not a popular type of plastic for toy lines I believe but its ability to retain plasticisers and be flexible enough for varied colours, moulds, etc...make it popular for everything from MLP to Barbie.