My Secret Unicorn: Starlight Surprise by Linda Chapman
Model Number: 9780241369197
Number 4 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...
There are rumours going round school that there is a haunted treehouse by the creek. After Lauren's brother's toy Donkey is thrown up there by the mean boys from school, it's up to Lauren and Twilight to get him back and to solve the spooky mystery. Little do they know, they're not the only ones in for a Starlight Surprise . . .
For ages approx. 8 years+.