Little is known about Rhun besides it is a Barbarian race that hates Gondor. The only visual representation of Rhun we have are the easterlings in the movie.(who have the flag of Haradrim thus no one knows who they really are.)
I suggest that we make Rhun a very Barbaric race that has a
few armored troops like the Easterlings from the movies.
Here are some ideas for regular troops
Wildmen of Rhun: Weak militia grade troops armed with clubs and furs.(like men of Dunland.)
Rhovanion Axemen: Men with Huge Axes and little armor.
Rhun Pike Militia: Like Wildmen of Rhun.
Ash Mountain Warriors: Rhun troops dressed in
ORC LIKE Armor. Very Dark and evil looking with a variety of Infantry weapons.
Rhun Tribesmen: Light horsemen
Rhun Plainsmen: Javelin throwing lightly armored troops
Tribal Hunters: Light Archers
Tribal Hunting Calvary: Light Horse Archers
Raging Bullmen: Unarmored Wildmen, with Rhun Bull painted on Bear chest. Really tall and holds HUGE two handed Berserking swords. May charge without orders. (Warcry Ability)(later end unit)
Heads of the Bull: Leather armored troops, with huge halberds. Halberds have a Bull skull on them.(later end unit)
Powerful Rhun Unit like Knights A.K.A Easterlings
Easterling Swords Men: Heavily Armored with sword and shield.
Easterling Spearmen: Easterlings with small shield and pike like spear.(black gate scene)
Easterling Archers: Heavy Archers with small swords.
Easterling Death Riders: Heavy Knight like Easterling Horsemen.
Easterling Flying Archers: Heavy Calvary Archers.
Generals Bodyguard.
Others:
Wainriders: Troops armed with spears (and maybe bows or Javelins) Riding in flat Chariots similar to those used by the Celts.
Variags: (recruitable if Khand is Captured) A mixture between heavy viking and mongols wielding large two handed axes.