The Black King was the leader of a gang of wild horses that ruled a valley called "Kokuōdani". They would trample to death anyone who entered their territory. A rival lord tricked a squadron of Raoh's into entering the valley, where they were killed in a stampede. Raoh, Reina, and Sōga went into Kokuōdani alone and witnessed Kokuō fighting off two tigers that had attacked a foal. Raoh confronted Kokuō and they fought briefly. Raoh used a pressure point to euthanize the suffering foal and earned Kokuō's respect. Kokuō and his fellow horses then joined forces with Raoh's army, with Kokuō becoming Raoh's personal steed. He then received the suffix Gō (号, Gō?) after his name.
The Black King is the strongest horse in the world, just because everything Raoh wants, deserves and takes is the best there is.