work animal vs fauna

work animal and fauna both are nouns.

Word	Noun	Adjective	Verb	Adverb
work animal	Yes	No	No	No
fauna	Yes	No	No	No

As nouns, fauna is a hypernym of work animal; that is, fauna is a word with a broader meaning than work animal:

Other hypernyms of work animal include animal, animate being, beast, brute, creature.

work animal (noun)	fauna (noun)
an animal trained for and used for heavy labor	a living organism characterized by voluntary movement
	all the animal life in a particular region or period