On the back of a horse: rode horseback to town; horseback riding. (adverb)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "horseback" in a sentence
"There was nothing highfalutin about her company manners -- it was by this homely phrase that he differentiated this Dede on horseback from the Dede with the office manners whom he had always known."
"Today, in Santander, where I just visited, the final statue of Franco on horseback is being taken down."
"Tinker's Horsefeathers Lodge specializes in horseback hunts for prairie grouse and pheasants."