...those people riding/controlling the Mumakil are Haradrim - people from Harad, also called Southrons (so I guess that Harad is somewhere south...). They were loyal to Sauron, basically, like the Easterlings. At least that's what my brain has dredged up from somewhere deep down.
Here follows a comparative illustration by the late Karen Wynn Fonstad (based on a drawing by Tolkien) that shows Arda before and after the Change of the World:
Harad forms a second continent, still connected to Northern Middle-earth in the Third Age. Note a strong resemblance to modern Africa. The Haradrim seem to be intended to be the ancestors of modern Arabian peoples. It is easy to imagine other Southrons who are darker-skinned Men of Far Harad. 'There are older and fouler things than Orcs in the deep places of the world.' - Gandalf the Grey, The Fellowship of the Ring