In zul'drak arena,while doing The Champion of Anguish you can steal mamoth from Vladof the Butcher.On mammoth you can hit mobs,and you receive exp from this.Unfortunately mammoth is only last 5 minutes,but this can be used to tank some elite mobs or easy grind non-elite.
All you need to do this.
1)Wait someone to start The Champion of Anguish
2)Stay near mammoth and when he is become mountable,mount it.
3)Enjoy your battle mammoth!
Proofpic http://i47.tinypic.com/2ia5383.jpg