"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."
With Rhaego apparently still-born and deformed, it is hard to say whether anything will come of this prophecy. It might be that the old women have no real power, and so it was always just false mumblings, but the old woman certainly seems frightened enough to believe her own words. It is possible that someone else than Rhaego is the "stallion who mounts the world". For example, perhaps the stallion is a mis-interpretation of the birth of Danaerys's dragons -- they certainly will "mount the world" in a sense, flying high above it with their terrible flame and great strength to conquer it for her. Or the male-dominated Dothraki culture could refuse to believe that the prophesied leader of the Dothraki will be a woman -- Daenerys herself.