There is the possibility that the tail problem was due in fact to another horse chewing on your horse's tail. This is common with young horses that are kept together. It is surprising that an older horse was victim of what is often a young horse problem. The best way to combat this may be to use a tail bag to protect the tail, or, in some cases, people will mix soap and some cayenne pepper and fish oil to put on the tail to make it taste bad and hopefuly keep the tail chewers away.