Shortly after they were defeated by the mountain of Caradhras, the Fellowship decided to go through Moria so that they could pass the Misty Mountains. One night on their way to Moria, they were attacked by a large pack of wolves. After a little while (and with a particularly powerful spell from Gandalf) they managed to push the wolves back, but in the morning they discovered something strange:
When the full light of the morning came no signs of the wolves were to be found, and they looked in vain for the bodies of the dead. No trace of the fight remained but the charred trees and the arrows of Legolas lying on the hill-top. All were undamaged save one of which only the point was left.
"It is as I feared," said Gandalf. "These were no ordinary wolves hunting for food in the wilderness. Let us eat quickly and go!"
The Lord of the Rings Book 2 Chapter 4: A Journey in the Dark
If they weren't "ordinary" wolves, what were they? I understand that they were probably servants of Sauron, but what was special about them that made their bodies apparently disappear? Where did they come from?