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]