equestrian (adj.)
"pertaining to or relating to horses or horsemanship," 1650s, formed in English from Latin equester (genitive equestris) "of a horseman, knightly," from eques "horseman, knight," from equus "horse" (see equine). As a noun, "one who rides on horseback," from 1786. The feminine form equestrienne is attested from 1848 (Century Dictionary calls it "circus-bill French"). An earlier adjective was equestrial (1550s).