I've got a reg. buckskin quarter horse gelding whose tail just isn't growing. He's about 7 years old. The lady we got him from said that colts had eaten it but it's been about two years and it isn't much better. He always rubs the top of it out and the rest doesn't get very long or thick. I've tried some antifungal and anti-itch stuff from the feed store. What else can I do? What could be causing this?