My Secret Unicorn: A Special Friend by Linda Chapman
Model Number: 9780241370858
Number 6 My Secret Unicorn series.
Most of the time, Twilight looks like an ordinary grey pony, but when Lauren says the words of a spell he transforms into a magical unicorn and together they can fly all over the world...
Lauren is almost sure that Moonshine is another secret unicorn who needs someone special 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 nearby. Lauren discovers that Michael longs to have a pony of his own. Could he be the special friend that Moonshine needs to become a unicorn?
For ages approx. 8 years+.