The Last Secret — Louise Cooper

Tamzin is trying hard to come to terms with the loss of Moonlight, whom she now knows is the Blue Horse, the kind spirit who is trying to protect her. Although the evil spirit of the Grey Horse has been quiet for some time, Tamzin is inexplicably drawn towards a rocky outcrop just off the coast and goes out in a little boat to investigate. Soon she realises it is a trap and the Grey Horse has succeeded in capturing her in his den. Her blue talisman isn't powerful enough to summon up the Blue Horse, until she spots a small fragment of blue glass - the missing piece of her bracelet. The Blue Horse and the Grey Horse face each other for the final time while Tamzin tries desperately to escape. With the help of the Blue Horse, her friends and her Nan, Tamzin reaches safety and the spirit of the Grey Horse is recaptured forever. And much to everyone's surprise and delight, Moonlight returns, a pony once more.