To make the characteristic high-pitched sound a horse makes; a type of vocalization between a neigh and a snort.
From Middle English whinnen, of imitative origin—the word mimics the actual sound horses make. Similar to how 'neigh' and 'snort' also came from attempting to recreate animal sounds.
A horse's whinny is actually one of their most important communication tools—different whinny patterns convey emotion, location, and even individual identity, meaning a horse's whinny is like their unique voice signature that other horses can recognize from far away.
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