The only mention of the origin of the name comes from an early draft of Appendix F to The Lord of the Rings, which states 'Of the Speech of Men of the East and allies of Sauron all that appears is múmak, a name of the great elephant of the Harad' (The Peoples of Middle-earth I 2, The Appendix on Languages). Given the connection of these creatures to the Harad, it seems reasonable to assume that the allies in question were the Haradrim. It should be noted that this comment did not survive to the published version of the Appendix, and so cannot be considered as definitive.