cotton rat vs gnawer

cotton rat and gnawer both are nouns.

Word	Noun	Adjective	Verb	Adverb
cotton rat	Yes	No	No	No
gnawer	Yes	No	No	No

As nouns, gnawer is a hypernym of cotton rat; that is, gnawer is a word with a broader meaning than cotton rat:

cotton rat: destructive long-haired burrowing rat of southern North America and Central America
gnawer: relatively small placental mammals having a single pair of constantly growing incisor teeth specialized for gnawing

Other hypernyms of cotton rat include rodent.

cotton rat (noun)	gnawer (noun)
destructive long-haired burrowing rat of southern North America and Central America	relatively small placental mammals having a single pair of constantly growing incisor teeth specialized for gnawing