That Ar-Adunaim are the Black Númenóreans from Far Harad comign back to Middle-Earth to conquer their birth rites. Mûmakil being of the greatest warbeast in Middle-Earth and also from Far Harad. It would natural of the Black Númenóreans to claim these beasts for war. Could import mûmakils from their southern colonies to their new domain in Middle-Earth.