devil dog vs leatherneck

devil dog and leatherneck both are nouns.

Word	Noun	Adjective	Verb	Adverb
devil dog	Yes	No	No	No
leatherneck	Yes	No	No	No

As nouns, devil dog and leatherneck are synonyms defined as:

Other synonyms of devil dog include Marine, shipboard soldier.

devil dog (noun)	leatherneck (noun)
a member of the United States Marine Corps	a member of the United States Marine Corps