Kodos are for orcs in Warcraft 3. The single orc rides the kodo while drumming on war drums, and occasionally throwing axes at enemies.
Forsaken should use crossbows, else mostly an 'undeadified' human style. Instead of gryphons they use bats.
Scourge consists mostly of skeletons, ghouls, abominations, necromancers, liches, undead nerubians, death knights, acolytes, banshees, ghosts and so on. Their mounted unit should probably be a death knight.
Vrykul are giant humans in a viking fashion, sort of. They use longboats. Ride red proto-dragons (more primitive than the normal red) and bears. They use crossbows and else look mostly furred and kinda Norse. They would not use firearms or cannons. Their elite is the Ymijar (undead), the Kvaldir strikes from sea, Val'kyr is the judges of the society (undead), Vargul is the mass undead soldier of the vrykul. The majority of the units should be living, thus not undead.
Nerubians would have different sizes of themselves and of course different kinds:
A normal one:
http://www.wowwiki.com/File:Nurubian.jpg
A vizier:
http://images3.wikia.nocookie.net/wowwiki/images/e/e1/WarCraft_Raneman009b.jpg
Their flyers:
http://images1.wikia.nocookie.net/wowwiki/images/c/c9/WarCraft_Raneman008b.jpg
Their units are named simply as 'warrior', 'seer', 'spider lord', etc. They would not use ranged weapons but instead use carrion swarms.
Burning Legion, they consist of many demons. I think I would simply list them for you:
* Annihilan, the pit lords
* Ered'ruin, the doomguards
* Gan'arg, stunted Mo'arg, serving as engineers
* Man'ari eredar, Commanders of the Legion
* Mo'arg, the felguard and Chief Engineers of the Legion
* Nathrezim, the dreadlords
* Sayaad, the succubi
* Shivarra, religious leaders of the Legion
* Tothrezim, Infernal craftsmen
* Wrathguard, man'ari eredar subspecies, elite warriors
* Satyr, demonic Night Elves
* Felbeasts, magician devourers
* Voidwalkers, living void hungering for energy to consume
* Imps, a mass of small pests
* Demonic hounds, giant two-headed dogs who breathe fire
* Void terror, two headed hound like creature with stalk eyes poking out from its neck.
* Fel orcs, the red orcs pumped up on demon blood.
* Felblood elves, the red blood elves pumped up on demonic fel.
Hope that helped.