My Secret Unicorn: Riding Star by Linda Chapman
Model Number: 9780241363096
Number 12 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...
When Twilight and Lauren discover a new unicorn named Currant, they do everything they can to get his owner, Grace, to say the Magic Turning spell which will give him his magic. But when it finally happens Currant has trouble getting to grips with it all, especially the flying. Can Lauren and Twilight help him become the rising star he is meant to be?
For ages approx. 8 years+.