Young Reddish Blonde Girl Outside With A Knight's Helmet And A Horse

A narrowing of the generic meaning "servant" to "military follower of a king or other superior" is visible by 1100. The specific military sense of a knight being a mounted warrior in the heavy cavalry emerges only in the Hundred Years' War. The verb "to knight", i.e. to make someone a knight appears around 1300, and from the same time, the word "knighthood" shifted from "adolescence" to "rank or dignity of a knight".
In this respect English differs from most other European languages, where the equivalent word emphasizes the status and prosperity of war horse ownership. Linguistically, the association of horse ownership with social status extends back at least as far as ancient Greece, where many aristocratic names incorporated the Greek word for horse, like Hipparchus and Xanthippe; the character Pheidippides in Aristophanes' Clouds has his grandfather's name with hipp- inserted to sound more aristocratic. Similarly, the Greek ἱππεύς (hippeus) is commonly translated "knight"; at least in its sense of the highest of the four Athenian social classes, those who could afford to maintain a warhorse in the state service.
An Equestrian (Latin, from eques "horseman", from equus "horse") was a member of the second highest social class in the Roman Republic and early Roman Empire. This class is often translated as "knight"; the medieval knight, however, was called miles in Latin, (which in classical Latin meant "soldier", normally infantry).
In the later Roman Empire the classical Latin word for horse, equus, was replaced in common parlance by vulgar Latin caballus, sometimes thought to derive from Gaulish caballos. From caballus arose terms in the various Romance languages cognate to the (French-derived) English cavalier: Old Italian cavaliere, Italian cavallo, French cheval, Spanish caballero, French chevalier, Portuguese cavaleiro, Romanian cavaler. The Germanic languages prefer terms cognate to the English rider: German Ritter, and Dutch and Scandinavian ridder.