The little fox was a supporting character in A Patch of Shimmering Light. This book tells its story:

As punishment for his and his grandfather's cruelty, a boy is changed into a fox. The curse will not be lifted until the small animal does good in the world.

But how can it be helpful when others are afraid of its sharp teeth and drooling smile?

Still, the lonely beast does all it can to be a good neighbor for the forest, sometimes with the aid of its beautiful bushy tail. Along the way, it makes unlikely friends and survives being hunted, hungry, and cold.