This is what I found supposedly said by someone in Firaxis:
"Well we had a really long talk about [the Mongols] and whether we should include them in this installment of the series. Eventually we started asking ourselves if it was just time to move in different direction completely and if maybe we could find a less 'controversial' alternative. The Mongols were known for committing mass rape and murder and just kind of being jerks {laughs}, and how would people feel if we included someone like Hitler in the game? We still wanted to have a nomadic horse people from the steppe though, and eventually someone suggested the Scythians led by Queen Tomyris, who most people never even heard about. I remember we were doing our research on her and Steven literally asked me "How have I never heard of this woman?" So yes, that really fit into the vision we wanted to have of showing off lesser known but still larger-than-life personalities. It was like the stars aligned, basically. Of course Genghis Khan can still show up as a great general, but I'll probably delete him if I ever get him." {laughs}"
Sorry for the post without vote, but if this story is true, it is simply and utterly ridiculous, and Firaxis should get a better historical consultant (mongols being known for rape, murder and being jerks ... oh my ... O think an historian died there), and not add Rome (massive killing on Carthage and abusive slavery), Greece (massive slavery), Spain (slavery, almost genocide on mesoamerica and inquisition), Aztecs (cannibalism, human sacrifices), France (genocide try ont the huguenots, massive killing in the italian wars and slavery), Germany (genocide during nazism, massive killing on hussites), England (massive slavery and killings throughout their colonies), USA (massive slavery and persecution on different minorities, including natives), Macedon (slavery and mass killings on razing cities), China (massive killings, slavery), Japan (massive killings during the wars against Korea or XXth century, forced prostitution, slavery), Brazil (almost genocide on paraguayans), Russia (slavery, mass kilings, persecution on minorities), Norway (massive killings, piracy), and so on. It is utterly ridiculous : every empire practiced rape, killings among others, and I'm so astonished I'm out of words ! It is acceptable to add vikings and violent colonialists but not the mongols !? I ... I'm speechless ... and sad ...