many horses
驫 centers on a large group of horses, conveying the idea of many horses together.
驫 is formed by tripling the horse radical 馬, a common Chinese character formation method to indicate a multitude or intensity of the base meaning. The exact historical development is uncertain, but the triplication clearly conveys 'many horses'.
Three horses 馬 stacked together vividly show a herd: many horses.
For ヒュウ, imagine the sound of many horses whinnying together: 'hyuu' echoes the reading ヒュウ.
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