From Longman Dictionary of Contemporary Englishbilly goatˈbilly goat noun [countable] HBAa male goat – used especially by or to children → nanny goat
Examples from the Corpus
billy goat• Then you can come here and see auctions, and people bringing everything from a billy goat to an elephant.• The oldest and biggest billy goat knocked the troll into the stream, and no one ever heard from him again.• Very soon the biggest billy goat came across the bridge.• The first billy goat makes just a little noise and has a tiny voice.• Soon the middle Billy Goat Gruff came across the bridge with a louder trip-trap sound.• The Characters Three billy goat brothers: Hungry. 