Mutos generally looked like deformed 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 labour.
Once an individual became mutated, he was forced out onto the wastes by his deformity-free comrades. Both sides drove Mutos out to maintain racial purity. The Thals used them for slave labour, particularly for dangerous jobs.
The 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.