Don't worry mate, you're not alone. I also wish to add the Mongols, and Spain, to my mod. After all, both Spain and the Mongols had a much greater impact on world history than the Aztecs or Iroquois.
I'm currently trying to do the graphics for both a Horse Archer AND a Mongol Horseman, but I'm new to this unit artwork lark. Give me about a week (or a fortnight.... or more!) and I should have something.
As you know, the Mongol conquest began in 1204 AD when Chingis or Genghis Khan united all the Mongol tribes. As this was the period when the mailed Knight was dominating western battlefields, I think the unit should come into the game with 'Chivalry' (by the way, dispite what civ3 says, Knights were used BEFORE Medieval Pikemen, not after! So I always swop Feudalism with Chivalry).
As for stats, I was thinking of making a Mongol Horseman a 4_3_3 with ZOC and only requiring horses (obviously!), and reducing the Chinese Rider to 4_2_3 with ZOC and requiring horses and iron (it did take the Mongols some 20 years to conquer China, so they weren't that much better), with both units costing about 70 to build.
I want the Horse Archer to be a 3_2_3 with ZOC, only requiring horses, costing about 50, and coming into play with say Monarchy. I'd like this unit to be available ONLY to Egypt/Babylon/Persians/Mongols INSTEAD of Knights, because these nations did NOT use Medieval western Knights in reality! (As you can see, I like my civ to be historicaly accurate, if possible). Do you think these stats look about right?
Anyway, less chat and more unit building. I'll be in touch.....