when he knew the fight was lost, what did Beauty Smith do to White Fang?

Beauty Smith beat him savagely.

In the meantime, the abysmal brute in Beauty Smith had been rising into his brain and mastering the small bit of sanity that he possessed at best.  When he saw White Fang’s eyes beginning to glaze, he knew beyond doubt that the fight was lost.  Then he broke loose.  He sprang upon White Fang and began savagely to kick him.