'Goat hawk' (or 'goathawk', 'goat-hawk') is a local or regional common (that is, not scientific) name for Caprimulgidae (from a Latin root meaning "goat milker"; see entry at Wordsense.eu) species. In the US, they are also known as whip-poor-wills.
Nightjar, Goatsucker, Dorhawk, Fernowl, Nighthawk, Churnowl, Goathawk, Wheelhawk, Wheelbird, Wheelowl, Spinningbird, Goat-milker, or Evejar.
Observations of the natural history of swallows, Thomas Forster, 1817.
The genus name (caprimulgus) derives from a mistaken folk belief that the birds suckled goats, causing the goats to cease producing milk.
A mysterious bird of night, bearing the sombre colours of the reed and the night upon his body, and bearing in his record the legend of goat-sucker, the etymology of which I think is at fault, unless, indeed, the goat-hawk moth was meant, and the bird originally called "goat-hawk sucker," and subsequently "goat-sucker;" but the Broadsmen know nothing of this widely-spread superstition.
Birds, beasts and fishes of the Norfolk broadland, P.H. Emerson, 1895.