bellow vs let out

bellow is a noun but let out is not a noun.

Word	Noun	Adjective	Verb	Adverb
bellow	Yes	No	Yes	No
let out	No	No	Yes	No

As verbs, let out is a hypernym of bellow; that is, let out is a word with a broader meaning than bellow:

Other hypernyms of bellow include emit, let loose, utter.

bellow (noun)	let out (noun)
a very loud utterance (like the sound of an animal)

bellow (verb)	let out (verb)
shout loudly and without restraint	make (clothes) larger
make a loud noise, as of animal	make known to the public information that was previously known only to a few people or that was meant to be kept a secret
	express audibly; utter sounds (not necessarily words)
	bring out of a specific state