My Secret Unicorn: Stronger Than Magic by Linda Chapman
Model Number: 9780241369203
Number 5 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...
On one of their evening fly-arounds Twilight starts to feel ill and he and Lauren have to stop exploring and return home. When it happens again they realise that Twilight's mysterious illness only flares up when he has used his magic to become a unicorn. Can they find something stronger than magic to help . . .?
For ages approx. 8 years+.