I have an Egyptian Arabian and my vet calls this HYPERSENSITIVITY. There is a little fly that causes a lot of horses to react by rubbing the top of their tail and sometimes their mane. It has nothing to do with electric fences or worms. One of the easiest ways to know if that is probably the case is to see if the horse does it in the winter. Hypersensitivity usually will begin in March, peak in June/July and diminish by the end of October. Our vets solution to this is Zyrtec since it is an allergic reaction. He recommends the generic version from Walmart, Sam’s or Costco. My horse is about 900lbs. and she gets 12 tablets in the a.m. and 12 in the p.m. It is simply put in a little grain. It has almost completely alleviated her tail rubbing although she still turns her lip up if I am willing to scratch it for her. Before she was diagnosed I would clip the top 5 inches of her upper tailbone, giving it a dressage clip. This made the appearance much more tolerable (otherwise her tail hair will stick out as though it had been burned with a match). Good luck, I hope this helps.