Making the characteristic high, loud sound that horses make.
From Old English 'hnægan', imitative origin representing the actual sound a horse produces; related to Old High German 'hneigon', true onomatopoeia.
Neighing is wonderfully imitative—in Old English it was 'hnægan' with that breathy H-sound that almost sounds like a horse breathing, proving the word tries to capture the actual sound.
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