The Collaborative International Dictionary of English v.0.48:

Neigh \Neigh\, n.
   The cry of a horse; a whinny.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Neigh \Neigh\ (n[=a]), v. i. [imp. & p. p. Neighed (n[=a]d);
   p. pr. & vb. n. Neighing.] [OE. neien, AS. hn[=ae]gan,
   prob. of imitative origin; cf. MHG. n[=e]gen, Icel. hneggja,
   gneggja, Sw. gn[aum]gga. Cf. Nag a horse.]
   1. To utter the cry of the horse; to whinny.
      [1913 Webster]

   2. To scoff or sneer; to jeer. [Obs.]
      [1913 Webster]

            Neighed at his nakedness.             --Beau. & Fl.
      [1913 Webster]

WordNet (r) 3.0 (2006):

neigh
    n 1: the characteristic sounds made by a horse [syn: neigh,
         nicker, whicker, whinny]
    v 1: make a characteristic sound, of a horse [syn: neigh,
         nicker, whicker, whinny]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "neigh":
   bark, bawl, bay, bell, bellow, blare, blat, blate, bleat, bray,
   call, caterwaul, cry, give tongue, give voice, howl, low, meow,
   mew, mewl, miaow, moo, nicker, pule, roar, screak, scream, screech,
   squall, squeak, squeal, troat, ululate, wail, whicker, whine,
   whinny, yap, yawl, yawp, yelp, yip, yowl