These are various species that have gone extinct in the animorphs universe.
“The Arn are an extinct avian race, original inhabitants of the Sector 5, RG-21578-4, and the...Arn
“The Nesk were a hivemind race of ant-like creatures that settled on Earth around 65,000,000 B.C...Nesk
“The Five are a sentient species who hunted Venber to extinction. The Five in turn, were than...The Five
“Graffen's Children were a primitive, peaceful alien race that were one of the many races that...Graffen's Children
“The Mercora were a peaceful race that came to Earth and colonized it around 65,000,000 B.C. when...Mercora