Here's why:
* "Animal" is a broad category: It includes everything from insects to whales, encompassing a vast diversity of life forms.
* Specific terms exist: While "animal" is the most general term, there are many other terms used to describe specific types of animals, like mammals, birds, reptiles, etc.
* "Fauna" refers to all animals in a specific region: This term is more focused on the animal life within a particular area.
Therefore, "animal" is the most accurate and encompassing noun for all living creatures that are not plants.