how does the gray cub gets his name of white fang?

The first Indian who approached him also named him.....

"One of the Indians arose and walked over to him and stooped above him. The cub cowered closer to the ground. It was the unknown, objectified at last, in concrete flesh and blood, bending over him and reaching down to seize hold of him. His hair bristled involuntarily; his lips writhed back and his little fangs were bared. The hand, poised like doom above him, hesitated, and the man spoke laughing, “Wabam wabisca ip pit tah.” (“Look! The white fangs!”)."