cavy vs guinea pig

cavy and guinea pig both are nouns.

Word	Noun	Adjective	Verb	Adverb
cavy	Yes	No	No	No
guinea pig	Yes	No	No	No

As nouns, guinea pig is a hyponym of cavy; that is, guinea pig is a word with a more specific, narrower meaning than cavy:

cavy: short-tailed rough-haired South American rodent
guinea pig: stout-bodied nearly tailless domesticated cavy; often kept as a pet and widely used in research

Other hyponyms of cavy include Cavia cobaya, aperea, Cavia porcellus, wild cavy.

cavy (noun)	guinea pig (noun)
short-tailed rough-haired South American rodent	stout-bodied nearly tailless domesticated cavy; often kept as a pet and widely used in research
	a person who is subjected to experimental or other observational procedures; someone who is an object of investigation