The Phantom Pony by Peter Clover
When Alice and her family move to an isolated hill farm, Alice longs for her own pony. And one day, she sees the most beautiful grey pony ever - shimmering at the edges, gleaming in the moonlight. Soon Alice and the pony go everywhere together. But is the pony real? For when Alice is given her own real pony, a chunky dark bay native pony, the beautiful grey seems to disappear. Until the night of the terrible snowstorm when Alice must ride through a virtual white-out to reach the village and fetch a doctor for her mother...