In “The Circus Train,” near the end of the third season, the ThunderCats experience major relief when the bounty hunter Captain Bragg locks up the Mutants and drags them off to the prison of Way Outback on his spacefaring locomotive. The villains had been Mumm-Ra’s most frequent allies, so their forced departure from the planet was certainly a blessing to all the people of the League of Third Earth.
The break from the vile creatures’ presence did not last long for the ThunderCats, however, as the Mutants are freed from their bondage soon after by Mumm-Ra. In “Return to Thundera, Part 4,” the mummified nemesis seeks their aid one final time in order to assault the feline heroes on two fronts: Third Earth and New Thundera. As expected, the plan ends in failure, for the Mutants prove again to be no match to the ThunderCats and before long they find themselves back in the cells of the bounty hunter.
Before the end of the series, the Circus Train not only makes it back to Third Earth, but also no longer holds the Lunataks since they are set loose by the maniacal pirate captain in the episode “Cracker’s Revenge.” So, even with ample opportunity to save them, both the Lunataks and Mumm-Ra decide to let the Mutants stay locked up on the traveling prison, which shows that the mummified dark lord has finally given up on them because of their ceaseless incompetence.