These are various species that have gone extinct in the animorphs universe.
“The Arn were the original inhabitants of the Sector 5, RG-21578-4. Masters of biological and...Arn
“The Five are a sentient species who kidnapped and harvested the Venber, rendering them extinct...The Five
“The Nesk were a hivemind race of ant-like creatures that settled on Earth around 65,000,000 B.C...Nesk
“The Mercora were a peaceful race that came to Earth and colonized it around 65,000,000 B.C. when...Mercora