From A Wiki of Ice and Fire
The Stallion Who Mounts the World is a Dothraki prophecy about a future figure who will rise to become the most powerful khal ever seen.
||"As swift as the wind he rides, and behind him his khalasar covers the earth, men without number, with arakhs shining in their hands like blades of razor grass. Fierce as a storm this prince will be. His enemies will tremble before him, and their wives will weep tears of blood and rend their flesh in grief. The bells in his hair will sing his coming, and the milk men in the stone tents will fear his name." The old woman trembled and looked at Dany almost as if she were afraid. "The prince is riding, and he shall be the stallion who mounts the world." 
In A Game of Thrones, Daenerys's son with Khal Drogo, Rhaego was believed to be the Stallion. He died in the womb before any part of the prophecy could be fulfilled.
||The stallion who mounts the world will burn no cites now. His khalasar shall trample no nations into dust.
- Mirri Maz Duur
, to Daenerys
References and Notes