Mûmakil (known in Westron as Oliphaunts) were large creatures resembling elephants, often used in battle by the Haradrim. Oliphaunt was the name given to them by Hobbits.
Presumably, Mûmakil could be coerced to kneel or lie down so that a tower could be hauled into place, and tied from under the belly. Their elevated position gave arrows and spears a greater range for those riding in the tower. The word Oliphaunt is a variant spelling of the archaic word oliphant meaning “elephant”, “ivory”, “elephant-tusk”, “musical horn made of an elephant tusk”, or “a musical instrument resembling such a horn.