Harsu and the Werestoat by Barbara Else
Harsu has five droplets of god blood and a treasured cloak to remember his father by. Now his father is gone, he lives with his mother, his only friend an old onager donkey. And something is not right with Harsu's mother. She has started kidnapping children, and sometimes her skin grows a soft down, little sharp ears emerge, and she turns into a horrible stoat. Harsu doesn't know if five godlet drops are enough to help him rescue the kidnapped children and turn his own mother to good. And now that he is twelve, he too might be becoming a were-animal ...