Cruel leader of the Rat people. He is one of Mumm-Ra's six generals.
The descendant of the one time Ratmen leader Ratilla, Ratar-O sought to claim the objects his ancestor once held, the Sword and Gauntlet of Plun-Darr, objects that once were owned by Mumm-Ra. To this end he started a slave labor operation in Mount Plun-Darr, using Thunderian slaves to search in the cursed mountain for the objects. Eventually some of the Thundercats, Lion-O, Panthro, Cheetara, Tygra and Pumyra infiltrated his mining operation posing as slaves. While Cheetara and Tygra retrieved the weapons, Lion-O, Pumyra and Panthro freed the slaves and fought the Ratmen. Lion-O ultimately defeated Ratar-O with some unexpected help from Mordax, Ratar-O's servant.
Ratar-O is a revamped version of the original 1980's Thundercats villain of the same name.