A horse that can run at a gallop, or a person who rides a horse at a gallop.
From gallop (from Old French galoper, possibly from Germanic roots meaning 'to leap') plus the agent suffix -er, meaning 'one who does the action.'
Medieval knights needed to identify their best gallopers for war, and horsemen became obsessed with breeding faster ones—this obsession eventually led to the modern thoroughbred racing industry we see today.
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