What happens to BUck after he discovers what happened to Thornton? Why do the Yeehats talk about Buck as a "Ghost Dog" and "Evil Spirit" at the end of the novel?

Buck mourns the death of Thornton very deeply. He was the only one who shared Buck's identity as a true creature of the Wild, and he was the only one who could ever legitimately claim to be his master in the first place. Buck is a Ghost Dog for the Yeehats because he single-handedly drove their entire tribe away, killing some of them. For them, the Ghost Dog is the ghost of the dead man Thornton.

What did the Yeehats

tell of a Ghost Dog that ran at the head of a pack?