belt bag vs butt pack

belt bag and butt pack both are nouns.

Word	Noun	Adjective	Verb	Adverb
belt bag	Yes	No	No	No
butt pack	Yes	No	No	No

As nouns, butt pack is a hyponym of belt bag; that is, butt pack is a word with a more specific, narrower meaning than belt bag:

belt bag: a small pouch (usually with a zipper) that attaches to a belt and is worn around the waist
butt pack: a waist pack worn with the pouch in back

Other hyponyms of belt bag include fanny pack.

belt bag (noun)	butt pack (noun)
a small pouch (usually with a zipper) that attaches to a belt and is worn around the waist	a waist pack worn with the pouch in back