I'd say get a short wig, and then a pile of extensions, wefts, or even loose wig fiber in the same color. It would probably be easiest to use red craft foam and wire to make the base of her ponytail, then glue the longer hair over it and attach it to the back of the short wig.
I don't think it's possible to make her hair entirely out of one wig >_>
Anime North 2013
Spitfire ☆ MLP:FiM ☆ At Crafter's Corner