river horse vs hippopotamus

river horse and hippopotamus both are nouns.

Word	Noun	Adjective	Verb	Adverb
river horse	Yes	No	No	No
hippopotamus	Yes	No	No	No

As nouns, river horse and hippopotamus are synonyms defined as:

river horse and hippopotamus: massive thick-skinned herbivorous animal living in or around rivers of tropical Africa

Other synonyms of river horse include hippo, Hippopotamus amphibius.

river horse (noun)	hippopotamus (noun)
massive thick-skinned herbivorous animal living in or around rivers of tropical Africa	massive thick-skinned herbivorous animal living in or around rivers of tropical Africa