Selena had never shown her beast to Xander, because the man didn't want to be marked by her so soon.
Xander didn't want Selena to mark him, because he knew the procession would be very painful and he wasn't sure if he could control himself, or rather control the fire inside him.
It wasn't pain that Xander was afraid of, but the huge potential that there was a chance he would burn Selena that made Xander reluctant to do so.
Therefore, when Selena suddenly ran and transformed into her white beast, Xander was so taken aback, he was shocked to his wits. His complete focus was sucked by the beautiful white beast alone.
Selena had said that the color of her beast was white, like snow, and Xander had imagined the girl in her white beast form, but he never imagined that Selena's wolf would turn out that beautiful.
The white beast was the most beautiful thing Xander had ever seen in his life.
