Totally making this up as I go, and assuming we're considering elephants as warrior mounts rather than transport:
1. War elephants are only likely to be used in battles between native forces, being too valuable (and the perch atop them too risky) to face massed European firepower;
2. A unit of War Elephants consists of 3 elephants with two warriors and a mahout atop each;
3. The warriors counts as cavalry (+2 in melee), the unit leader and the mahouts are key personnel, with the elephant itself removed when the mahout is;
4. The unit always counts as a Class IV target vs fire, to represent the robustness of the beasts themselves versus musket fire or bows;
5. Due to their terrifying nature, non-elephant units must re-roll declared charges against elephants and stand-vs-charge when attacked by them, and the elephant unit does not shed stragglers when charging.
Maybe something like this?