Mardrut

The Mardrut is an oceanic creature that lives in the oceans of one of the moons of the Andalite homeworld.

Physiology
The Mardrut is the size of a whale with dark red skin and hundreds of little thrashing 'fish tails' to keep up momentum. Its moves through the ocean by shooting water out of three massive water sacs attached to its underside.

Source

 * Animorphs #4: The Message.