The Mongols have the Keshik as their unique unit (replacing the Horse Archer). The Keshik is very much like the Horse Archer (+50% attack with catapult, requires Horses resource and Archery and Horseback Riding technologies). In addition, the Keshik ignores terrain penalties (making it very useful in Jungle, Tundra Forest and Hills) and has 1 First Strike. Overall, the unit is not giving you a serious advantage – but is a serious threat to enemies' Archers (even in cities).
The starting technologies for the Mongols are Hunting and the Wheel. This means that Archery and Animal Husbandry can be researched straight away – leading to Horseback Riding, which provides the Keshik. Pottery is also just one tech away (with Agriculture or Fishing).
The civilization has 2 leaders – Genghis Khan and Kublai Khan.