Equestrian

google

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mid 17th century (as an adjective): from Latin equester ‘belonging to a horseman’ (from eques ‘horseman, knight’, from equus ‘horse’) + -ian.

wiktionary

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Ultimately from Latin equester(“of or pertaining to equestrians/cavalry”) + -ianus(“ -ian, related to”, adjective marker), from equus(“horse”).

etymonline

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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" (from PIE root *ekwo- "horse"). 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).