horse vs horseback

horse and horseback both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse	Yes	No	Yes	No
horseback	Yes	No	No	Yes

horse (noun)	horseback (noun)
solid-hoofed herbivorous quadruped domesticated since prehistoric times	the back of a horse
a chessman shaped to resemble the head of a horse; can move two squares horizontally and one vertically (or vice versa)	a narrow ridge of hills
a framework for holding wood that is being sawed
a padded gymnastic apparatus on legs
troops trained to fight on horseback

horse (verb)	horseback (verb)
provide with a horse or horses

horse (adverb)	horseback (adverb)
	on the back of a horse