Caballus

♦ In Classical Latin this word denoted an inferior riding horse or even a packhorse, but from the sixth to the tenth or eleventh centuries it denoted a good horse, usually a warhorse. In Southern France and Spain it retained this meaning, but in North-Western Europe, Italy and Germany it was superseded by other words such as equus or destrier. It is rarely found in England at all. From it are derived the normal words for "horse", "knight" and "chivalry" in French, Italian and Spanish (e.g. cheval, chevalier, chevalerie and cavallo, cavaliere, cavalleria). The English words "cavalier" and "chivalry" are derived not direclty from the Latin but from the Italian and French respectively, occuring first in 1560 and c. 1590.

(Davis, R.H.C. The Medieval Warhorse: Origin, Development and Redevelopment, 135)