Slightly bad update, I rehaired a pony in the Custompony.com hair, and it seems to be some kind of polypropylene. It's water-resistant, stiff, and the ends started getting rough/damaged just from the brushing I did while I was rehairing her. It takes a cold curl (which poly usually won't), so it may be a very high-quality poly, or some kind of nylon I've never heard of, but it's definitely not interchangeable for MLP nylon. The good news is that the 'finisher' that was on the hair washed off, so now my fingers don't get tacky from handling it anymore!