A thought, if you are worried about moisture, they do make things that absorb moisture - I think people use them in places like bathrooms or basements? You could put some of them in their with them to help. Maybe not too close, but enough that if there was a lot of excess it might help.
I also learned that however the pony's hair looks when you store it, it will stay that way! LOL So wrap up hair carefully on the neck so that it doesn't stick out when you take them out. Tails I twirl and pull forward in between the leg to keep them from getting frizzy.