Linda Chapman 
My Secret Unicorn: A Special Friend [EPUB ebook] 

Lauren is almost sure that Moonshine, the scruffy grey pony at the dealer’s stable, is another secret unicorn who hasn’t yet found a unicorn friend to turn her into her magical shape. Feeling sorry for the lonely pony, Lauren pays her a visit and is surprised to see a strange boy talking softly to Moonshine and stroking her. Michael explains that he loves ponies but his foster parents won’t let him have one because, among other things, they live in the city. Lauren and her unicorn, Twilight, are certain that Michael is the special friend that Moonshine needs and are determined to unite them before the summer holidays end and Michael has to return home.