The Mongol Army is a recurring antagonistic faction in the animated series Phineas and Ferb, serving as the secondary antagonistic faction of the episodes "Lotsa Latkes" and "Doof Dynasty".
The Mongol Army first appeared when Heinz Doofenshmirtz accidentally summoned them to the present with his Historical Army Retrieve-inator. Doof is upset by this as he intended to summon the Spartan Army, so he heads off to buy a Genghis Khan costume. In the meantime, the Mongol Army came across the Potato Gremlins and pursued them into the Danville Retirement Home, where Doof finally catches up to them. However, Perry uses the inator to send the Mongol Army, along with Doof and the Potato Gremlins, back to the Mongol era. It is likely that the Mongol Army have killed and ate the Potato Gremlins while Doof builds another inator to get back to the present.
It was told the Mongol Army was under the leadership of Doofus Khan, as the latter is known for his countless attempts to take over the Tri-Province Area. In 1542, Doofus sends several of his Mongol warriors to pose as acrobats into capturing Princess Isabella and hold her hostage, which they do so successfully, much to Doofus' delight.
Fortunately, the 1542 Phineas and Ferb, along with the 1542 Baljeet and Buford, were taught by Master Perry to learn the "Way of the Platypus". With their new skills, the boys were able to easily defeat the Mongol Army before rescuing Princess Isabella and defeating Doofus. It is likely that the Mongol Army would be arrested alongside their master for their crimes and eventually passed away.