The wainriders were a collection of easterling tribes, united by their hatred of gondor who raided Rhovaniaon and Gondor over the 3rd age.
They rode in great wagons and chariots, faster than almost all other cavalry, successfully using guerrilla tactics to take all Gondorian land east of the river Anduin. They used composite bows and spears in battle, riding their chariots and skirmishing their enemies to death from afar.
They were nomads and had little houses or settlements other than camps.
They could be added in a rhunic overhall update like the harad one we are getting.