brown rat vs wharf rat

brown rat and wharf rat both are nouns.

Word	Noun	Adjective	Verb	Adverb
brown rat	Yes	No	No	No
wharf rat	Yes	No	No	No

As nouns, wharf rat is a hyponym of brown rat; that is, wharf rat is a word with a more specific, narrower meaning than brown rat:

Other hyponyms of brown rat include sewer rat.

brown rat (noun)	wharf rat (noun)
common domestic rat; serious pest worldwide	brown rat that infests wharves
	someone who lives near wharves and lives by pilfering from ships or warehouses