A Madness of Kentaurs

It is the time of year when once wild things become wild again, still wild things become wilder, and civilized things shut their doors and pretend they had never been wild. During the season of madness, the kentaurs of the Pelion foothills-those half-horse, half-human creatures that humans call "centaurs"-journey from their home villages to the sacred lands across the Acheron River. Horses find the kentaur herd irresistible and often run away from their human masters to join the herd.
One day, Octavian, stable-boy and floor cleaner at the local inn, gets carried off by the innkeeper's horse, and ends up trapped in the kentaur herd. He is placed under the charge of Ixion, a young kentaur shaman-in-training, who begins to teach him that kentaurs are not the barbaric, half-sentient beasts he had thought they were. And then the dreams begin. Dreams in which ancient, cold things with too many teeth steal the boys' tongues and threaten to steal their sanity.
Now Octavian and Ixion both have to learn enough about the otherworld to make it though a grueling initiation ceremony across the Acheron, River of Woe. But first, Octavian has to gain acceptance from the kentaur herd.