also: あくめ · あくうま
noun
wild horse; unmanageable horse
Refers to a horse that is difficult to control or ride, often with a nuance of being unruly or vicious.
彼は悪馬を手なずけるのに苦労した。
He struggled to tame the wild horse.
あの悪馬には近づかないほうがいい。
You'd better not go near that unmanageable horse.
暴れ馬 emphasizes a horse that is actively bucking or rampaging, while 悪馬 focuses on a generally unmanageable or vicious nature.
Compound of 悪 (bad, evil) and 馬 (horse). The exact historical derivation is uncertain, but the word straightforwardly describes a horse with a bad temperament.