The Great Turtle is one of the Divine Beasts.
Katia can face the Turtle after finding the Phaorah's Tomb in the Temple of Amentankh. It fights alongside two Sphynx and is the only one of he Beasts to do so.
As a card
Whilst the Turtle has respectable Defense and HP for a Summon, it's attack has pitiful range. This gives the Monster extreme durability but is only really good at close-range combat rather then long. This could still be handy for immobile foes, however, such as a Crystal Rose.