An Eight-rout Army man riding a horse come from the front 为什么不用 is riding

riding a horse 做定语修饰 Army man

riding a horse 是现在分词短语作后置定语修饰前面的名词，但你可以将它变成定语从句，即 ：
An Eight-rout Army man who rides a horse comes from the front.

谓语是come
riding a horse是修饰主语的

这个句子的主干是army come from the front，riding在这里是后置定语！

riding 等作定语而is riding 不可为定语