In Arabye, þei ben clept Gerfauntz; þat is a best pomelee or spotted .. but a lityll more high þan is a stede, But he hath the necke a xxti cubytes long. [Mandeville's Travels, c.1425]
The modern form of the English word is attested by c.1600 and is via French girafe. Replaced earlier camelopard, a compound of camel (for the long neck) and pard (n.1) "leopard" (for the spots).