Thinktank made a mod about Oglala Sioux. It's a bit similar to Sauromatia, with lots of cavalry and lots of archers including some with poisoned arrows.
Here's a link to the DomII version.
Post#333881
I'm not sure if it works. It uses the old way of modding national spells, for instance, replaces Tien Chi, replaces Tien Chi national spells etc.