Can the Mumak itself be your army leader? How about the Chieftain controlling it?

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

to the best of my knowledge the mumakil itself can not, (if its too stupid not to be able to use a banner i think its too stupid to lead a force),but the chieftan is a hero so the chieftan can lead your force (in SBG, but not too sure about WotR)

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By the blood of our people are your lands kept safe. Give Gondor the weapon of the enemy. Let us use it against him.