NOUN (2)

1. hair taken from the mane or tail of a horse;

2. a fabric made from fibers taken from the mane or tail of horses; used for upholstery;

The Collaborative International Dictionary of English v.0.48:

Horsehair \Horse"hair`\, n.
   A hair of a horse, especially one from the mane or tail; the
   hairs of the mane or tail taken collectively; Specifically: a
   black shiny fabric made of such hairs, used commonly in the
   1800's as a covering for stuffed furniture such as chairs and
   couches.
   [1913 Webster +PJC]

   Horsehair worm (Zool.), the hair worm or gordius.
      [1913 Webster]

WordNet (r) 3.0 (2006):

horsehair
    n 1: hair taken from the mane or tail of a horse
    2: a fabric made from fibers taken from the mane or tail of
       horses; used for upholstery

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "horsehair":
   bristle, capillament, catgut, chord, cilium, coat, fiddlestring,
   fleece, fur, hair, mane, nylon string, pelt, pile, pubescence,
   pubic hair, setula, shag, steel string, string, wool,
   wound string