The Veleek is an amorphous alien creature that resembles a cloud of dust, but can form blades and teeth to attack its prey with, as well as arms, wings, and even functioning eyes on a whim. It isn't strictly one creature, but billions of tiny insect-like flying creatures that act as one and can take on any shape they want. Individually, they're the size of a flea, with hundreds of wings, upturned "bowls" that act as antennae, and two catfish-like filaments stretching out beneath their bodies. These beings feed on energy, which they absorb by draining it from their victims using the filaments.
It was discovered by the Yeerks on Saturn, who after attacking it (unsuccessfully) with wave after wave of Hork-Bajir and Taxxons, they decided that it would be a useful weapon against the "Andalite bandits". According to Visser Three, the word "Veleek" means "pet" in the Yeerk language. He tells Ax that he named it this himself.
The Veleek could not be taken as a host, so it was controlled by mutating it. The mutated form was attracted to morphing energy and would seek out and attack those it sensed morphing, even though it could not absorb this energy. It could only absorb energy from the Blade ship's engines, and even then only what the Yeerks granted it, thus forcing it to serve them. The Veleek's one weakness was water, which disrupted the individual creatures from acting as one and apparently dissolved them. However, it also had problems with weight, being unable to carry Rachel while she was in elephant morph.
When dispatched against the Animorphs, the Veleek managed to capture Ax, but the remaining Animorphs swiftly realized that the Veleek had a key weakness; since it was drawn to morphing energy, all they had to do was keep morphing to keep it occupied, as it wouldn't continue to pursue a morphed target if another person was morphing nearby. Having learned of its weakness to water, the Animorphs defeated it by luring it out over the sea, while Tobias carried Cassie above the Veleek in cockroach morph, Cassie then jumps and subsequently morphs a humpback whale, thus causing the Veleek to fall into the ocean below and dissolving it.