Assuming the Mumbles are actually undead mummies, these Mumbles were once humans, possibly the previous residents of the castle. At some point earlier, someone, possibly the Thing, wrapped the Mumbles in a cloth-like material throughout their entire body so that only their small eyes were visible. The rest of their body was a pale, white cloth. They were also set in a stage of hibernation in which they could only be awoken if a human stepped too near it.