Brad-
it is tempting to think that could be an elephant ridden in the Indus Valley towards ATG's romp across the Orient but I believe it is a Persian war elephant. Indian armies of the time, especially on the elephants, were lightly armored, if at all. Mainly as a result of the heat and humidity. To me, the leggings are a dead give away as is the decorative cloak over the head of the elephant. It will be a show stopper for sure.
Regarding the "Plains Indian"- I am happy to be wrong but it looks to me more like a mongol horseman- Atila the Hun era.