The Nestorian Christians who rode with Chingis Khan were Kerait Mongols. Their chief, Wang Khan, had made a treaty of friendship with Temujin's father, Yesukai. This had entitled Temujin to look upon him as a guardian after his father's death, but during the tribal wars the old man had betrayed him and it was not until after the bloody battle of Gupta and the death of Wang Khan that the Karaits joined the armies of Temujin. It was this Wang Khan that Marco Polo later identified as Prester John; he had never been to Europe, but he was the only Christian ruler of an eastern people, and, just as the word Mongol became Mogul in India, so, as it passed from language to language, the name Wang came to sound like John. Although Chingis Khan could not therefore be describes as Prester John's grandson, he could be described as his ward.
from page 36, The Devil's Horsemen