eques

Eques, equitis, m., third declension, from equus: a man on a horse. The stem equit- is invisible in the nominative, which catches learners out every time.

In the field the plural equites is the standard word for cavalry as an arm of the army, set against pedites, the foot. In politics it names the equestrian order, the propertied class ranked below the senators, who took the name from the public horse the state once supplied for their service and kept it long after the horse stopped mattering; by Cicero's day an eques is a financier rather than a rider. Medieval Latin uses the word for a knight. English gets equestrian, and equine from the horse itself.