"The Nesk are scavengers. The ships they fly, the weapons they use, are all modeled on the tools of races the Nesk have defeated. The Nesk have learned to mimic the bodies and shapes of these other races in order to fire the weapons and fly the ships. The Nesk believe the dinosaurs belong to them. As their property. They believe this planet belongs to them. But they cannot tolerate the existence of other sentient, intelligent species. They are determined to wipe out the Mercora."
The Nesk were a hivemind race of ant-like creatures that settled on Earth around 65,000,000 B.C. They had the ability to shape themselves into different forms in order to operate weapons and machinery.
During the late Cretaceous period the Nesk immigrated to Earth. The Nesk warred with the Mercora, a peaceful race who had also settled on Earth. The Nesk believed that Earth belonged to them, as did the dinosaurs, and wished to eliminate any other intelligent or sentient species they came across, including the Animorphs and the Mercora. After the Animorphs successfully stole a nuclear weapon from their base, the Nesk evacuated the planet, although not before redirecting a comet towards Earth. The comet ultimately killed off the dinosaurs and the Mercora, while the Animorphs managed to use the nuclear fallout of the comet's impact to return back to their time. It is unknown what exactly happened to them, but they may have eventually evolved into the simple modern-day ants of today.