1
on horseback
固定搭配，horseback 为名词，整个介词短语作方式状语，意为“骑着马”。
We went through the mountains on horseback.
我们骑马穿越了群山。
在马背上；骑在马上
先理解核心含义，再看它在真实句子里的用法。
在马背上；骑在马上
on the back of a horse
We went through the mountains on horseback.
我们骑马穿越了群山。
在马背上
On the back of a horse.
We explored the valley on horseback.
我们骑马探索了山谷。
She has never travelled on horseback before.
她以前从未骑马旅行过。
The tourists enjoyed a guided tour on horseback through the countryside.
游客们骑马享受了一次有导游的乡村之旅。
固定搭配，horseback 为名词，整个介词短语作方式状语，意为“骑着马”。
We went through the mountains on horseback.
我们骑马穿越了群山。
horseback 在此为副词，直接修饰 ride，相当于 ride on horseback。
She learned to ride horseback at a young age.
她很小就学会了骑马。
Not this
We traveled by horseback through the mountains.
Use this
We traveled on horseback through the mountains.
表示“骑马”这一方式时，英语用固定搭配 on horseback，不用 by horseback；by 通常用于交通工具（如 by car、by train），而骑马是骑在动物背上，要用 on。
Not this
He rode on the horseback to the farm.
Use this
He rode on horseback to the farm.
on horseback 是固定短语，意为“骑着马”，horseback 前不加冠词或所有格。说 on the horseback 是照搬汉语“在马的背上”导致的多余冠词错误。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
骑在马上
They explored the national park on horseback.
他们骑马游览了国家公园。
骑马运动；骑马
Horseback riding is good exercise.
骑马是一项很好的锻炼。
骑马者；骑手
A horseback rider must wear a helmet.
骑马者必须戴头盔。
骑马之旅；骑马游览
We signed up for a horseback tour of the canyon.
我们报名参加了峡谷的骑马游览。
骑着马；在马背上
They explored the countryside on horseback.
他们骑马探索了乡村。
Use Horseback naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
horse（马）+ back（背）→ 马背；想象骑马时坐在马背上，就是 horseback。
语域：neutral 常见场景：travel、equestrian riding、historical description horseback 常见于 on horseback 这一搭配，表示“骑在马上”；副词用法如 ride horseback 也存在，语气与介词短语接近。整体通常视为中性语域，比较自然地用于旅行、骑马或叙述性文字，口语和书面均可用。建议优先掌握 on horseback 的固定搭配。
horseback 作名词时通常出现在 on horseback 中；作副词时本身表示“骑在马背上”，无需介词。
Dictionary facts from ECDICT, WIKTIONARY.