The Collaborative International Dictionary of English v.0.48:

Nylghau \Nyl"ghau\, Nylgau \Nyl"gau\, n. [Hind. & Per.
   n[imac]lg[=a]w, prop., a blue cow; Per. n[imac]l blue +
   g[=a]w cow. See Lilac, and Cow the animal.] (Zool.)
   A large Asiatic antelope (Boselaphus tragocamelus, syn.
   Portax tragocamelus), found in Northern India; called also
   the blue bull. It has short horns, a black mane, and a
   bunch of long hair on the throat. The general color is
   grayish brown; the male is blue-grey with white markings, and
   the female is brownish with no horns. [Written also
   neelghau, nilgau, nylghai, and nylghaie.]

   Syn: nilgai, nylghai, blue bull, Boselaphus tragocamelus.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

Boselaphus tragocamelus
    n 1: large Indian antelope; male is blue-grey with white
         markings; female is brownish with no horns [syn: nilgai,
         nylghai, nylghau, blue bull, Boselaphus
         tragocamelus]