underling vs bottom dog

underling and bottom dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
underling	Yes	No	No	No
bottom dog	Yes	No	No	No

As nouns, bottom dog is a hyponym of underling; that is, bottom dog is a word with a more specific, narrower meaning than underling:

Other hyponyms of underling include associate, cog, man, second banana, second fiddle.

underling (noun)	bottom dog (noun)
an assistant subject to the authority or control of another	a person of low status