protoceratops vs ceratopsian

protoceratops and ceratopsian both are nouns.

Word	Noun	Adjective	Verb	Adverb
protoceratops	Yes	No	No	No
ceratopsian	Yes	No	No	No

As nouns, ceratopsian is a hypernym of protoceratops; that is, ceratopsian is a word with a broader meaning than protoceratops:

protoceratops: small horned dinosaur
ceratopsian: any of several four-footed herbivorous dinosaurs with enormous beaked skulls; of the late Cretaceous in North America and Mongolia

Other hypernyms of protoceratops include horned dinosaur.

protoceratops (noun)	ceratopsian (noun)
small horned dinosaur	any of several four-footed herbivorous dinosaurs with enormous beaked skulls; of the late Cretaceous in North America and Mongolia