My Secret Unicorn: Twilight Magic by Linda Chapman
Model Number: 9780241370896
Number 10 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...
Snooty Jade Roberts is jealous when Lauren makes friends with the new-born foal at the stables, and she is determined to cause trouble. But when Jade gets into danger, only Lauren and Twilight can help. Will Twilight's magic be enough to save her?
For ages approx. 8 years+.