The Stone Guardian is one of the guardians of the Lost Pyramid of Sandopolis. Towering way above normal people, this defender of treasure is virtually indestructable and can repair itself quickly if smashed to pieces. Despite the guardian probably being built by the ancient Echidna race, it did not recognise Knuckles the Echidna when Captain Plunder summoned him and sentenced all that disturbed him to death. After punching Sonic the Hedgehog into the ground, the guardian was lured into a massive pit of quicksand by Knuckles, condemning him to a life under the ground.
A very similar guardian was later seen deep in the pyramid, guarding the treasure room. Nack the Weasel had studied the ancient texts and devised a way to free the guardian and set it upon Knuckles and the Chaotix Crew. Although fairly smaller than its predecessor, it proved to be just as invulnerable and was only defeated when Knuckles used the shrinking ray that Nack had left behind.
The Stone Guardian was also the Sandopolis Zone sub-boss in the Sonic & Knuckles game. A similar boss appeared in Sonic Adventure 2, under the control of Doctor Robotnik.