The Kaarks were an advanced race native to the planet Rhoos. They resembled humanoid chickens, possessing red-brown skin, claw-like hands, long necks, no chins, and red crests. They were led by a Supremo.
The Kaarks were forced to live underground when Fo crashed on their planet and began ravaging it, killing or kidnapping its inhabitants. Their weapons useless against him, all they could do was observe his movements. One such group, led by Ff'ni, discovered the First Doctor and his companions near Fo and captured them, taking them on board their spaceship. After finding that Fo had left the area, they took them to the Kaark Supremo, who told them of their world's plight. The Doctor went on a mission to retrieve his ship from Fo's stronghold, rescued his companions, and slew Fo. (PROSE: The Playthings of Fo)