Stymphs are magical creatures on the side of evil.
In the Books Edit
A stymph is a bird that has just a skeleton. Sophie and Agatha rode on a stymph up to The School Master's Tower. Also, when Sophie needed to get to Agatha's room, she stole one's egg, and when it attacked she rode it to Agatha's room. It is clear that stymphs hate Nevers.
Later, it is noted that the School Master trained the Stymphs to attack Evil.