Little Thunder's Secret

Little Thunder is quite an astounding pony, and he and Yakari truly are the best of friends. So when upon waking up one morning, he discovers that his mount has left the village, the young Sioux boy is devastated. He tries to find his friend, but is forced to give up as the trail grows cold: whatever it is that drives Little Thunder, he's heading due south as fast as he can. What is it that calls out to him?