My Secret Unicorn: Twilight Magic By Linda Chapman

A new adventure starring Lauren and her magical pony, Twilight who turns into a beautiful, snow-white unicorn when Lauren says the magic words.
There's great excitement at the stables - a beautiful mare called Apple has just had a foal! The new foal is gorgeous and seems to be watching Lauren with a very wise look in his huge eyes. Lauren's excitement is spoiled when snooty Jade Roberts turns up. Jade goes to Lauren's school and used to be Twilight's owner but she sold him because he was too quiet and dull. Lauren, of course, knows better...
That night Lauren and Twilight visit the foal in secret. Suddenly Twilight hears a noise and they find Jade up to no good. Apple, fiercely protective of her foal, is frightened and angry at Jade's intrusion. Even though she doesn't like Jade, Lauren knows she has to do something to help. Lauren and Twilight make a daring rescue to save Jade who has to admit that she may have been wrong about Twilight after all. Later, Lauren and Twilight discover the reason for Apple's over-protectiveness - that maybe, just maybe, Currant is a new unicorn, but they'll have to wait at least a year to find out.