The mogu were originally dire trolls of the Zandalari tribe.
Here are some of the things we know about the Mogu:
* The Mogu originated as a race of wild brutes which stumbled into magical waters of the Vale of Eternal Blossoms. (Dire trolls fit the description of "wild brutes".)
* The Mogu were known to have strong ties with Zandalar Tribe before the Sundering and were tight allies during the war for Pandaria. (Zandalar trolls do not ally with non-trolls, so it would make sense for the Mogu to have troll origins.)
* Unlike "lesser races" that need schooling in the arts of magic, the ability was innate in the Mogu. (Night elves are descended from trolls that discovered the Well of Eternity; that process of uplift gave night elves powerful, innate magical abilities.)
* The Mogu often preferred to use slaves to build their massive monuments. (Goblins originally lived as slaves to the trolls; it's possible that the trolls had more slave races.)
* The Mogu were continually at war with the mantid, and built the Serpent's Spine all to contain them. (The mantid may have been an aqir race, and as such, mortal enemies of the trolls.)
All of this is speculation and circumstantial at best, but I think it's a reasonable theory until we get more information.