holler vs yowl

holler and yowl both are nouns.

Word	Noun	Adjective	Verb	Adverb
holler	Yes	No	Yes	No
yowl	Yes	No	Yes	No

As nouns, holler and yowl are synonyms defined as:

Other synonyms of holler include bellow, bellowing, holla, hollering, hollo, holloa, roar, roaring.

holler (noun)	yowl (noun)
a very loud utterance (like the sound of an animal)	a very loud utterance (like the sound of an animal)
a small valley between mountains