river vs River Cocytus

river and River Cocytus both are nouns.

Word	Noun	Adjective	Verb	Adverb
river	Yes	No	No	No
River Cocytus	Yes	No	No	No

river (noun)	River Cocytus (noun)
a large natural stream of water (larger than a creek)	(Greek mythology) a river in Hades that was said to be a tributary of the Acheron