Pinworms (Oxyuris equi): Yes, these are often the culprit behind tail rubbing. Pinworms are more common in younger horses, but can occur in any age of horse. The female worms lay their eggs around the horse’s anus and these eggs cause the itching. You cannot see them with the naked eye, but your vet can use clear tape placed over the anal area and then a microscope to see if eggs are present. (You can likely do this, too if you have a good microscope and know what to look for.)