The March of the Old Gods
The Plainsmen have inhabited the Plains of Athenry for as long there have been Humans in the region, and have simply never ceased their nomadic lifestyle like the men of Amarith have. The plainsmen never settle in an area for more than a few months, and simply pack up and move when game becomes scarce, or when threats become too dangerous to continue to stay in the area.
The plainsmen and the men of Amarith have had full on battles in the past, but as soon as the orcs unite, or giants begin to come down from the mountains, they will set aside their differences temporarily and fight the menaces that threaten their ways of life. For the most part, relations between the two groups are mostly peaceful.
The plainsmen are highly skilled horsemen, and their horses are their lives. They are trained to ride horse shortly after they are able to walk on their own, and from there they are trained to fight, hunt, and even sleep on horseback.