What explanation did Duncan give for the strange cry Ch 7

He says it is a horse in agony,

"Here, then, is one who can undeceive you,” said Duncan; “I know the sound full well, for often have I heard it on the field of battle, and in situations which are frequent in a soldier's life. 'Tis the horrid shriek that a horse will give in his agony; oftener drawn from him in pain, though sometimes in terror. My charger is either a prey to the beasts of the forest, or he sees his danger, without the power to avoid it. The sound might deceive me in the cavern, but in the open air I know it too well to be wrong.”