The Garatrons are a race that that look very similar to Andalites, but they have no blade on their tail, a head that resemble a bike racer's helmet, and no stalk eyes on top. They seem to be a case of convergent evolution (where two unrelated species evolve into similar forms.) Garatrons are able run at speeds that make a cheetah look like nothing, they're so fast, they can't be seen, and can even run on water. Unlike other known species, Garatrons even use a form of extremely fast thought-speak that is hard to understand until after a few seconds.
The Garatrons appeared in Book #37: The Weakness, when the Animorphs, minus Jake, faced an Inspector from the Council of Thirteen, who came to Earth to see why Visser Three was taking so long to conquer the planet. The inspector was later bitten and poisoned by Marco in his cobra morph and his enchanced metabolism accelerated the spreading of the poison and killed him. The Garatrons were supposed to be a new Controller race, but were never seen again after the Inspector's death - on Earth anyway. Little else is known about them.