One of the big problems with the Crayola stuff (or any other air dry paper mache material) is that is shrinks as it dries. You'll end up having to reattach it to the pony with super glue of something similar once it dries completely. That probably means internal support armatures would be a bad idea -- the clay would just crack off the wire as it dries. Also, if it gets wet it will start to break down. And it was really hard to make a smooth surface. And it deforms under its own weight on large pieces while it's drying. Also, there is NO WAY you could use up all 2.5 lbs of the Crayola before the carton dries out on you if you're just making horns and whatnot.
Yikes, this sounds completely negative, but I do speak from experience. It was good for practice sculpting, but I wouldn't use it on a pony I wanted to sell or give to someone else. If I could go back in time seven years, I'd have bought something else to do Plummy's legs.