My Secret Unicorn: The Magic Spell by Linda Chapman
Model Number: 9780241369166
Number 1 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 Foster is excited about moving to her new home on a farm, especially now that her parents have promised that she can get a pony of her own! When she meets Twilight at a pony auction, she knows immediately he's the one for her. But there's more to Twilight than meets the eye. With the help of the magic spell she finds in an old book, can Lauren unlock the secret and help Twilight become who he was born to be?
For ages approx. 8 years+.