The Thals, along with the Kaleds and the Mutos are one of the three native sapient species of the planet Skaro. It is unknown whether the Thals and Kaleds were different factions of the same race or different races.
The Thals and the Kaleds fought a bitter centuries-long war over possession of the planet, reducing much of its surface to rubble in the process. At the apex of the war, the Thals prepared to fire a nuclear weapon, the intention being to completely wipe out their hated enemy (along with the innocent Muto population.) The Time Lord called "The Doctor" intervened, and the launch was aborted, but the Kaled bunker was destroyed, trapping the Kaled offshoot race known as the Daleks underground for thousands of years. During this time the Thals evolved into a strange species of pacifist farmers.
The Thal are distinguished from the Kaleds by their blonde hair and light eyes (Episodes: The Daleks, Planet of the Daleks, Genesis of the Daleks).