Name the internal parasite associated with tail rubbing.

The internal parasite associated with tail rubbing is pinworms. Pinworms are small, white parasitic worms that inhabit the intestines of horses and are known to cause irritation around the anal area. This irritation can lead to intense itching, causing the horse to rub its tail against various surfaces in an attempt to relieve the discomfort.

In contrast, roundworms mainly affect younger horses and can cause a range of gastrointestinal issues rather than external signs like tail rubbing. Tapeworms are typically associated with colic or digestive disturbances and do not usually cause the specific tail-rubbing behavior. Threadworms primarily impact foals and are less commonly associated with tail rubbing than pinworms, which are specifically known for causing such irritation. Thus, pinworms are clearly the internal parasites linked with this behavior in horses.