Veleek

The Veleek is a creature from Saturn in Animorphs.

Physiology
The Veleek is an amorphous alien creature that resembles a cloud of dust. The Veleek 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. Together with incredible coordination and teamwork they can form blades and teeth to attack prey. The Veleek feeds on energy, which it absorb by draining from their victims using the filaments.

The Veleek's one weakness was water, which disrupted the individual creatures from acting as one and apparently dissolved them. In addition it had a limit of how much weight could be carried even with the combined force of the creatures that make up the Veleek.

History
Discovered by the Yeerks on Saturn, who after several unsuccessfully attacks and numerous sacrifces of Hork-Bajir and Taxxons, managed to capture it. The Veleek could not be taken as a host, but interest in weaponizing it against the "Andalite bandits" reamined. The Yeerks mutated the Veleek, conditioning it to be attracted to morphing energy. The Veleek would track and capture the Andalites, bring its prey to the Blade Ship, where it would received sustenance from the energy of the Blade ship's engines as it could not absorb the morphing energy.

When dispatched against the Animorphs, the Veleek managed to capture Ax. 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. After Ax escaped and relayed the Veleek's weakness was water, the Animorphs defeated it by luring it out over the sea, while Tobias carried Cassie above the Veleek in cockroach morph, Cassie then jumped and subsequently morphed a humpback whale, thus causing the Veleek to fall into the ocean below and dissolving it.

Source

 * Andalite's Gift