Who or what does Buck find in the forest in the middle of the night? What happens upon making this discovery?

Buck discovers a timberwolf in the forest.... they make friends, and they play.

But in the end Buck's pertinacity was rewarded; for the wolf, finding that no harm was intended, finally sniffed noses with him. Then they became friendly, and played about in the nervous, half-coy way in which fierce beasts belie their fierceness.