goat vs billy goat

goat and billy goat both are nouns.

Word	Noun	Adjective	Verb	Adverb
goat	Yes	No	No	No
billy goat	Yes	No	No	No

As nouns, billy goat is a hyponym of goat; that is, billy goat is a word with a more specific, narrower meaning than goat:

goat: any of numerous agile ruminants related to sheep but having a beard and straight horns
billy goat: male goat

Other hyponyms of goat include kid, billy, he-goat, nanny-goat, nanny, she-goat, Capra hircus, domestic goat, wild goat.

goat (noun)	billy goat (noun)
any of numerous agile ruminants related to sheep but having a beard and straight horns	male goat
a victim of ridicule or pranks