racer vs greyhound

racer and greyhound both are nouns.

Word	Noun	Adjective	Verb	Adverb
racer	Yes	No	No	No
greyhound	Yes	No	No	No

As nouns, greyhound is a hyponym of racer; that is, greyhound is a word with a more specific, narrower meaning than racer:

racer: an animal that races
greyhound: a tall slender dog of an ancient breed noted for swiftness and keen sight; used as a racing dog

Other hyponyms of racer include steeplechaser, finisher.

racer (noun)	greyhound (noun)
an animal that races	a tall slender dog of an ancient breed noted for swiftness and keen sight; used as a racing dog
slender fast-moving North American snakes
a fast car that competes in races
someone who drives racing cars at high speeds