So called because it was brought to us from the coast of Guinea, where it is very common.
“Notwithstanding their barsh cry ... I like the Guinea-fowl. They are excellent layers, and enormous devourers of insects.” —D. G. Mitchell: My Farm of Edgewood. chap. iii. p. 192.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Guinea Fowl from Infoplease: