The Giant Turtle was a very old (judging from its size) turtle that lived in the Black Mud Swamp. It has red scaly skin and glowing red eyes. Its teeth appear to be pointed and it might be carnivorous like most fresh water turtles, though this is unconfirmed and it might also be omnivorous or herbivorous. Its shell is covered with soft moss that is brown like soil or like mud and there is a colony of Red Willows that grow on it in a spiral pattern with their leader at the center of the spiral. Many birds also live on the turtle's back.
A Red Willow colony once started to grow on its back and their leader emitted a sleeping gas that made the turtle fall asleep. Its back appeared as an island to the ones who lived in the Black Mud Swamp and they called it the Black Island. However in its sleep the turtle could move its head, which would appear as a monster to the nearby creatures.
The Xylith, a plant that the Undines use to make a transparent material from which they build the bubbles in which they live, grows on the back of the turtle and one of the Undines would sometimes volunteer to go on the island to collect some Xylith leaves to bring back to the Undines' underwater village/town although it was risky because of the "monster".
In The Monster and The Willow, when the Flora went to the Black Mud Swamp to prepare for her exams, she and the Winx met the Undines. The Undine Queen, Algae, told the Winx that they need the Xylith to make their homes and that it grows only on the Black Island but that it is dangerous to go there because of the monster, so one of them would occasionally risk going to the island to collect some Xylith and that the last volunteer, Lusiz, never returned.The Winx decide to go to the island to defeat the monster and save Lusiz and on their way to the island Stella suggests that maybe the monster is actually a normal-sized creature that appears as a large and terrifying monster to the small Undines. The head of the sleepwalking turtle then comes out of the water and unknowingly catches Musa in its mouth. The Winx then use a water bubble spell to go underwater and Bloom jumps in the turtle's mouth and saves Musa. However they have to crash land on the island because the turtle's head is moving again.
On the island they Winx find a colony of Red Willows and fall asleep, except Tecna, because of the sleeping gas emitted by the leader Red Willow. Tecna creates a magical shield and wakes up the Winx, who help her expand the shield and wake all the other sleeping creatures on the island. They find Lusiz at the foot of the leader Red Willow and she explains that the "monster" is in fact a giant turtle that is sleeping because of the Red Willows' gas and moves its head in its sleep. The leader Red Willow then attacks the Winx but Bloom and Flora defeat it and free the turtle and all other sleeping creatures on the turtle's back.
The turtle then allows the Undines to pick as many Xylith leaves as they want before swimming away since it is now free and Flora uses a rapid-growth formula she invented to make many more Xylith leaves grow for the Undines so that they will never run out of Xylith and will always be safe in their bubbles.