He got to be the dirtiest dog in town.
And the easiest time for an animal to tell is the time to stop work and eat.

Felix was very clever in that regard.

At about six o'clock the unsuspecting Whitey dismounted to stretch himself and ease the strain of jouncing up and down on that rocking-chair that had come to feel like a ridge-pole.

Naturally his eyes turned away from Felix, to whom he was beginning to take a personal dislike.
Whitey's eyes were brought back with a jerk by the soft thud of little hoofs on the prairie, for Felix was beating it back toward Willer Bend, with a speed that astonished his late rider.

Whitey started after him instinctively, but he soon realized that that was useless, and he stood and watched, while Felix became a blurred spot in the distance.