Stone Guard is the large muscular stone beast that appeared in the episode The Ancient One.
Leonardo travels to Japan, sent there by Master Splinter, in order to train with the Ancient One. Traveling the path that will take him to the Ancient One's temple atop a mountain, Leonardo encounters a strange little fat man who has a taste for chocolate and is extremely flatulent. Leo accepts the man's offer to show him a shortcut and travels with the man to the mountain top. When they reach the temple, stones begin to fly through the air until they form a huge Stone Guard. The stone beast attacks and Leo fights for his life until he finally slashes away the stones covering the creature's face. The stones fall to reveal a mirror image of Leonardo himself. Leo then realizes that the anger he has been punishing himself with is pointless and that he has been his own worst enemy. He and the Stone Guard bow to one another, and then the stones fall to the ground. Leo then turns with the realization that the strange man is the Ancient One.