The Mongol hordes were the armies of Genghis Khan and his successors that invaded and ruled much of medieval Russia from the early thirteenth to early fifteenth centuries. The word "horde" is Tater in origin and describes any nomadic group of people (interesting side note: The false spelling, "Tartar", comes from "Tartary" which was the medieval European term for the steppes and deserts of Central Asia. This area became known as "Tartary" because it was thought to be similar to the Greeks' "Tartarus", one of the lower levels of Hades). Genghis Khan's invasion force employed many Tater cavalrymen, who identified the whole group as a "horde". Thus, word trickled back to mainstream Europe that an invading horde was coming.
When this "horde" arrived and defeated the European armies by virtue of their superior training and tactics, the miltary establishment was dumbfounded. After all, they were Europeans, they were Christians, the only way they could be defeated was by sheer overpowering numbers. Eventually it was decided that this was the case and any returning soldiers or fleeing civilians were simply suffering from "shellshock" if they reported otherwise. So, in the end the Tatars' word "hordes" was corrupted to mean "a massive group of people" and Europeans continued in the delusional belief that they were naturally superior to everyone else.