Fred Diamond has a simple set of free wargames rules for playing the Indian Wars in the America of the late 19th century.
I’m currently painting some of Wargames Foundry’s Plains Indians for a 7th Cavalry game. I found this site, with descriptions of symbols that the Native American warriors painted on their horses.
Mark Ainsworth offers a set of simple solo wargames rules for games of the 7th Cavalry against Plains Indians.
Tombstone is a free RPG set in the wild west. It’s designed to be rules-light, and has quick play character creation to get you started.
Black Hat, White Hat is a set of free wargames rules for Wild West skirmishes.