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 method of movement through the ocean is shooting water out of three massive water sacs attached to its underside. Though it isn't terribly fast, it never tires, but also can't turn well.

Source

 * Animorphs #4: The Message.