You know, there are other things you can do to help the situation.Maybe the mammoth gets exposed to a disease and becomes ill. They have to go on a quest to find a cleric with a spell that will cure him (normal cure disease works on humanoids, not elephants?)
Maybe the beastie starts looking at every large hairy object as a potential mate? They realize it is time to find a girl mammoth...
It is only an animal. It should be afraid of things that animals are afraid of. Fire, undead, other freaky stuff may send it on a fear rampage, if the character involved fails an animal handling check.
I'm not saying destroy the usefulness of the monster, but cause it to have other repercussions on the characters' lives.
Also, what do the townsfolk think of this monster? Do they feel threatened when the PCs are around? What are they feeding it? How much does that cost? Where does the food come from? Does it require rare foods or supplements?
Have fun with it! Make it an object of roleplay and quirkiness, so fighting isn't the only reason they remember having the beastie...
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