What was the name of the Kereyid khan who was Genghis Khan's ally?

The Kereyid khan who was Genghis Khan’s ally was Wang Khan. He is also known as Toghrul or Ong Khan, and “Wang Khan” was his Chinese-derived title.

Toghrul ruled the Kereyid confederation in the region of modern Mongolia. He had been an ally and blood-brother of Temüjin’s father, Yesugei, and later became an important patron of the young Temüjin. Their combined forces defeated the Tatars, helping Temüjin build influence among the steppe tribes.

Their relationship eventually broke down. Temüjin’s growing power threatened Toghrul’s position, while Toghrul’s son, Senggüm, opposed a closer alliance. In 1203, the Kereyids fought Temüjin and were defeated. Toghrul was killed while fleeing, and the Kereyid realm was absorbed into the emerging Mongol Empire.

A common mix-up is calling Wang Khan a Mongol khan. He was specifically the khan of the Kereyids, although his alliance with Temüjin was a major step in Genghis Khan’s rise.