她从马背上掉下来摔死了。

She was killed in a fall from a horse.

《牛津词典》

他从马背上摔了下来，但他却安然无恙。

The horse threw him off, but he got away in a whole skin.

《新英汉大辞典》

他用鞭子抽打马背。

He lashed the horse across the back with his whip.

《新英汉大辞典》

They used even a horseback relay that could go a hundred kilometers a day.

采用了马背接力的形式，一日可以行进100千米。

耶鲁公开课 - 新约课程节选

There are these enormous statues. One of the statues is the President Theodore Roosevelt on a horse, on horseback.

有很多巨大的雕塑。其中一个雕塑是西奥多•罗斯福总统骑在马背上。

美国的博物馆教育 - SpeakingMax英语口语达人