Aquila, the horse of the dark forest

As 16 year old Cally Brown was walking her dog Millie on a late moon lit evening, she makes an amazing discovery but no one will believe her.
Cally could not even believe her own eyes, a lone black mare had caught the eye of this young girl.
But where did she come from? What's her history? Why is she a danger and why is everyone afraid of her? Cally had soo many questions building up in her head. She must find out about her and she must gain her trust. As Cally asks everyone she knows about this horse, she then discovers a ghostly tail. This is going to be much harder than she thought to gain the young mares's trust.
Will she be able to prove everyone wrong and be able to touch, train and save this ghost horse?
She has only her 6 week summer holidays to save this mare before her time is up.