You may want to include some form of representing the Mongol invasion of the 1240's. Russian principalities were continuously struggling against states such as the Golden Horde until the late 1500's. The Mongols also invaded and devastated Poland, Lithuania, Hungary, and even fought with the Teuton Knights. To represent them, you may want to give them a couple of cities between the Ural mountains and the Caspian Sea, as well as a tech tree only they can access. The tech tree should be short and result in an UU based on the Keshik. Make it relatively cheap, requiring only horses, give them some bombard power to represent mounted archers and very fast. This way, the mongol kingdom would grow in importance quite qucikly once they acquire this unit. However, you should make them completely technologically backwards otherwise and lacking in culture, so they have great problems in a) keeping citizens happy; b) making money; c) doing scientific research. Thus, while very dangerous, they can eventually succumb to more advanced civs.
I suggest calling them The Golden Horde, making the UU 4/1/3 (3)
, cost 40 shields, require horses only (these stats would make sense in the epic game).