Actually it does not matter the campaign difficulty level. Carthage gets a unit of forest elephants in their army in Sicily. Just see the descr_strat file.

It makes a big difference if the human is Carthage or if you are plaing Romans on VH battle difficulty. The former makes the Carthaginian blitz strat possible by using the elepant as mobile ram for spectacular smashing ability up and down the Italian penisula. The latter makes certain that the ele actually wouldn't rout from falme arrows before hitting your battle lines and will actually plow through. However, with reasonable tactics, the ele will still die relatively quickly.