Horsehair is the horse tail hair and horse mane hair. Horsehair as well as the "tuft' at the end of a cow's tail are the same sources . There are other animals that have the same type of slick hair, but horses' tails are the only ones to have fibers long enough to weave with without spinning.

Horse Tail Hair: Double drawn in the color of white, black, gray, brown and mixed; single drawn in the color of white, black, gray, brown and mixed; refined horse tail hair.
Specification and measurement(inch):
4"-5" , 6"-8", 9"-12", 13"-15", 16"-17", 18"-21", 22"-26