The Silver Horse — Elizabeth A. Lynn

Seeing the Silver Horse as a cute toy, Susannah gives it to her brother, Niall, as a present. One night Susannah awakens and finds neither her brother nor the Silver Horse; racing to the park, she sees her brother riding not a toy, but a stunning stallion. Susannah, Niall, and the horse are whisked away to a land unseen and unknown. This is how this unforgettable adventure through fantasy and lost toys begins. Now, lost in the Realm of Dreams, Susannah must fight for her brother or he will forget his entire life as a human.