They're always in pairs of two so there's always the theory that they're statues of Nald and Thal, but even then, for a civilization as egotistical as the Mhachi were, I'd imagine they'd envision Nald and Thal as lalafells if they really did have a wholly lalafell population.

We can fairly safely say that least some of the Mhachi were Hyur because of those statues and the zombie black mages in various places. My baseless speculation is that there was a fairly large Highlander population within Mhach and that perhaps some of them fled to the highlands of Gyr Abania during the flood and later went on to found Ala Mhigo similarly to how Belah'dia was founded by survivors of the flood.