According with Lorewalker Cho in thee new patch... (link)
- The mogu were children as well. Children of the Titans. They were once a legion of stone, heartless and obedient. By the Titans' command, they fought the terrible servants of the Old Gods.
- They shaped the mountains and carved the rivers of the land. And they created a magical cradle of life in a hidden valley that we now call the Vale of Eternal Blossoms.
- But eventually the Titans fell silent. And their creations were cursed with flesh. The mogu grew restless. Many generations later, when Thunder King united them, they seized upon their legacy! I truly believe now that the mogu thought they were doing the work of the Titans. They fought against the mantid and used the powers of the Vale to create new life. Ah, but such terrible works! Parents cannot always be assured of the legacy they will leave behind. How especially true this is, when the parents are Gods. And their children - monsters.