It's a mandolin.

Yes, John Paul Jones played the mandolin on “The Battle of Evermore.” It’s the reason that song sounds more like a medieval folk ballad than a rock tune.

Jimmy Page has said the mandolin scratching in the background was recorded by Jones after Page brought the idea from a mandolin player he saw at a folk club. Jones didn’t really play mandolin before, but he figured it out in a few takes. The strange, plucky sound you hear is exactly that.

If you were expecting something weirder like a theremin or a hurdy-gurdy, no — just a mandolin. But for Led Zeppelin, that was unusual enough.