Prophet of Theos. The Aethon Chronicles, #4

A land in conflict. A perilous uprising to decide its fate. Titus, a young orphan stallion, lives among a band of thieves stealing to survive in his divided homeland: Aethon and Archippus. But when a devastating tragedy leaves him reeling, he is summoned by the Sovereign Creator to confront the rulers of Archippus for corruption. He obeys, only to have his warnings ignored and an endless darkness fall on the entire realm.
Hunted by the vengeful king and queen, Titus is forced into hiding. Meanwhile, hatred and strife drive the horses further apart, unaware that their enemies the cevervisteeds threaten to invade. With the fate of his kind resting on his back, Titus has no choice but to follow Theos's cryptic guidance on an uncertain path. The only path that can reunite and restore Aethon to its one true destiny.