When are the two times White Fang barks?

White Fang barked at Weedon Scott's horse, and he also barked at a porcupine.

When it reared, the master put the spurs to it and made it drop its fore-legs back to earth, whereupon it would begin kicking with its hind-legs. White Fang watched the performance with increasing anxiety until he could contain himself no longer, when he sprang in front of the horse and barked savagely and warningly.

Though he often tried to bark thereafter, and the master encouraged him, he succeeded only once, and then it was not in the master’s presence. A scamper across the pasture, a jackrabbit rising suddenly under the horse’s feet, a violent sheer, a stumble, a fall to earth, and a broken leg for the master, was the cause of it. White Fang sprang in a rage at the throat of the offending horse, but was checked by the master’s voice.