Xiongnu, Scythians, Huns.
I've created preliminary versions of each in my Delenda Est Git repository here: https://github.com/JustusAvramenko/delenda_est
Obviously the nomadic civs should have some similar attributes.
No territory bonus or restrictions
Consequently, their buildings have -50% health and loyalty
Raiding and Looting bonuses
A focus on cavalry
Cavalry units can build
Livestock bonus, called "Pastoralism."