'Mutos' was a derogatory term for those mutated by nuclear radiation in the war between the Thals and Kaleds.
Biology
Mutos generally looked like lumpy Thals or Kaleds, but the mutations could vary widely. The Mutos lived in the wilds of Skaro, trying to scrape out an existence in the radioactive wastelands and were often hunted by Thals for use as slave labor.
History
Once an individual became mutated, they were forced out onto the wastes by their formed comrades. They were hated by both sides. The Kaled mutos were driven out to maintain racial purity among the ranks. The Thals used them for slave labour, particularly for dangerous jobs.
The Mutos whose heads were uncovered all or mostly displayed dark hair, implyuing they were originally of Kaled stock. Since the Thal mutations eventually led full circle (The Daleks) it is possible Thals were more genetically stable and the majority of Mutos were Kaled. This is supported by the thals not having the same facist society with the focus on genetic purity, which the Kaleds developed in direct response to the Mutos.
Thals mistook Sarah Jane Smith for a Muto when they found her in the wastes. After finishing their Neutronic missile, the Thals had Mutos load it without protection from the radiation used in its construction.
Sarah Jane then led a revolt in an attempt to escape. The Mutos climbed up, through the hanger. Several escaped but most were shot while ascending. (DW: Genesis of the Daleks)