Sir Robert Holmes, in 1666, captured in Schelling Bay 160 Dutch
sail, containing bullion and
gold-dust from Cape Coast Castle in Guinea. This rich prize was
coined into gold pieces, stamped with an elephant, and called Guineas
to memorialise the valuable capture. (See
Guinea. The legend is M. B. F. et H. Rex. F. D. B. L. D. S. R. I. A. T. et E.—Magnæ Britainniæ, Franciæ, et Hiberniæ Rex; Fidei Defensor; Brunsvicensis, Lunenburgensis Dux; Sacri Romani Imperii Archi Thesaurarius et Elector.
Guinea-pieces = 21s. were first coined in 1663, and discontinued in 1817. The sovereign coined by Henry VII. in 1480 was displaced by the guinea, but recoined in 1815, soon after which it displaced the guinea. Of course, 20s. is a better decimal coin than 21s.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894