The Stallion Who Mounts the World is a prophecy in the Dothraki religion. According to the prophecy, the Stallion is the "khal of khals" who will unite all the Dothraki under his single khalasar and ride to the ends of the earth. The prophecy states that "he will be the khal of khals, and all the people of the world will be his herd."
Daenerys Targaryen's unborn son, Rhaego, was announced by the dosh khaleen to be the prophesied figure. However, her son was killed in the womb before any part of the prophecy could be fulfilled. The Lhazareen maegi Mirri Maz Duur believed that Rhaego would bring about unbridled death and suffering (much as Rhaego's father had visited upon her village), so she tricked Daenerys into sacrificing his life for the life of her husband, Khal Drogo.