The big one from an old campaign setting I helped make were called Pale Dancers. They were small, fleshy blobs of gray meat with perfect osteokinesis, as long as it or a bone it controlled was in contact with what it wanted to move, to the point of liquefaction. They'd make ambulatory bodies out of bones and then mindlessly wander around grabbing more and more. Small ones would make good midlevel party encounters, aiming to pierce the body and tear out the skeleton in a coup-de-grace if they could, and can kill the weak (read; non-player) instantly.
The problem is that they don't die of old age, and they have no limit to the amount of bone they can shape, as long as it remains in contact. The largest one still living is the size of Rhode Island and floats through the oceans as a living vessel for the silicate-based species living on it, eating whales and krakens. The largest one ever recorded ate the corpse of a god and became the moon.