bass horn vs euphonium

bass horn and euphonium both are nouns.

Word	Noun	Adjective	Verb	Adverb
bass horn	Yes	No	No	No
euphonium	Yes	No	No	No

As nouns, euphonium is a hyponym of bass horn; that is, euphonium is a word with a more specific, narrower meaning than bass horn:

bass horn: the lowest brass wind instrument
euphonium: a bass horn (brass wind instrument) that is the tenor of the tuba family

Other hyponyms of bass horn include bombardon, helicon.

bass horn (noun)	euphonium (noun)
the lowest brass wind instrument	a bass horn (brass wind instrument) that is the tenor of the tuba family