racehorse vs horse

racehorse and horse both are nouns.

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

As nouns, horse is a hypernym of racehorse; that is, horse is a word with a broader meaning than racehorse:

racehorse: a horse bred for racing
horse: solid-hoofed herbivorous quadruped domesticated since prehistoric times

Other hypernyms of racehorse include Equus caballus.

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

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