Before the Mongolian invasions, armies were intimidated when they faced an army larger in than themselves. Why did the people in neighboring regions learn to fear Genghis Khan's relatively small armies?

Genghis Khan was ruthless and very efficient when conducting scare tactics. His army was on horses, nimble and ready to surprise attack with swift strikes and to launch a hail of arrows while on the move. But most importantly he would not wage war just with soldiers but would also take hostages, torture and kill women and children and a lot more to scare his enemies into submission without too much fighting.