The Untamed by Max Brand

Buck and his father were learning of a thousand crimes charged against Dan. Wherever a man riding a black horse committed an outrage it was laid to the account of this new and most terrible of long riders. Two cowpunchers were found dead on the plains. Their half-emptied revolvers lay close to their hands, and their horses were not far off. In ordinary times it would have been accepted that they had killed each other, for they were known enemies, but now men had room for one thought only. And why should not a man with the courage to take an outlaw from the centre of Elkhead be charged with every crime on the range? Jim Silent had been a grim plague, but at least he was human. This devil defied death.

These were both sad and happy days for Kate. The chief cause of her sadness, strangely enough, was the rapidly returning strength of Dan. While he was helpless he belonged to her. When he was strong he belonged to his vengeance on Jim Silent; and when she heard Dan whistling softly his own wild, weird music, she knew its meaning as she would have known the wail of a hungry wolf on a winter night. It was the song of the untamed. She never spoke of her knowledge. She took the happiness of the moment to her heart and closed her eyes against tomorrow.