The Collaborative International Dictionary of English v.0.48:

Billy goat \Bil"ly goat`\
   A male goat. [Colloq.]
   [1913 Webster] billyo

WordNet (r) 3.0 (2006):

billy goat
    n 1: male goat [syn: billy, billy goat, he-goat]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "billy goat":
   billy, boar, bubbly-jock, buck, bull, bullock, chanticleer, cock,
   cockerel, doe, doeling, dog, drake, entire, entire horse, gander,
   goat, gobbler, hart, he-goat, kid, mountain goat, nanny,
   nanny goat, peacock, ram, rooster, she-goat, stag, stallion, steer,
   stot, stud, studhorse, tom, tom turkey, tomcat, top cow, top horse,
   tup, turkey gobbler, turkey-cock, wether