noun
unruly horse; unmanageable horse; unbroken horse
Refers to a horse that is wild, difficult to control, or not yet tamed. Often used in literary or descriptive contexts.
彼は悍馬を手なずけるのに苦労した。
He struggled to tame the unruly horse.
その悍馬は誰も乗りこなせなかった。
No one could ride that unmanageable horse.
暴れ馬 is a more common and colloquial term for a violent or rampaging horse, while 悍馬 is a literary term emphasizing wildness or unruliness.
Compound of 悍 (fierce, wild) and 馬 (horse). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning of an untamed horse.