equestrian:  Equestrian was adapted from Latin equester, an adjective derived from eques ‘horseman’. Eques in turn was based on equus ‘horse’ (source of English equine ). This was the Latin descendant of *ekwos, the prehistoric Indo-European term for ‘horse’, which was once found in all the daughter languages of Indo- European except for the Slavic branch: Old English had eoh, for example, Old Irish ech, Sanskrit avás, and ancient Greek híppos (source of English hippodrome and hippopotamus).
It is a remarkable circumstance, however, that over the past thousand years equus and its relatives have (other than in derivatives such as equine) died out, to be replaced by secondary terms such as French cheval (from Latin caballus, probably a non-Indo-European borrowing), German pferd (from late Latin paraverēdus ‘extra post-horse’, source also of English palfrey), and English horse. => equine, hippopotamus
"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).