A file of men ahorseback passed by.    From Wordnet, Princeton University. 

Men ahorseback and Aiel Maidens; not common companions.    From Wordnik.com.  [A Crown of Swords]

On the trail she was a mosquito afoot, a jockey ahorseback.    From Wordnik.com.  [Gigolo] Reference

He was unaware of the machinery of orders that started the charge, although from the corners of his eyes he saw an officer, who looked like a boy ahorseback, come galloping, waving his hat.    From Wordnik.com.  [The Red Badge of Courage] Reference