A Mardrut is an oceanic creature that lives in the oceans of one of the moons of the Andalite homeworld. It is the size of a whale with dark red skin and hundreds of little thrashing 'fish tails' to keep up momentum. It moves by shooting water out of three massive water sacs attached to its underside, which makes a threatening WHUMP, WHUMP, WHUMP sound. Though it isn't terribly fast, it never tires. It also can't turn well. Visser Three morphs this creature in Book #4: The Message.