Third Earth is implied to be our Earth in its distant future. Mumm-Ra and his pyramid are said to date from the time of the First Earth. Seemingly being Ancient Egyptian in origin. What happened in the intervening years is not revealed.
Humans seem to have survived. The Warrior Maidens (Amazons) seem to be entirely human. Their leader Willa and her sister Nayda are arguably the most prominent humans in the series. Queen Tartara of the Crystal Kingdom also seems human. Dr. Dometone, a male scientist, is also human. Curiously a couple of other human characters are not native to Third Earth. One is Mandora, the Evil Chaser. She is basically a "space cop", working for the Interplanetary Control Force. The other known human is Safari Joe, an" intergalactic big-game hunter". Which might indicate there are other humans living away from their native planet.
Mumm-Ra and his good counterpart Mumm-Rana are probably undead human beings. Several other immortals humans have been depicted: Brodo, Ta-She, and Wizz-Ra. All sorcerers who have been exiled from the Earth but have been able to briefly return. Wizz-Ra, Mumm-Ra's ancient enemy, has claimed to be able to return once every 7000 years. Which probably indicates how old he and Mumm-Ra are.The Arthurian sorcerer Merlin has also been depicted as still alive but with pointed ears reminiscent of an elf. The samurai Hachiman was once summoned by Mumm-Ra from the distant past. He seems to keep time-traveling between his era and the Third Earth on various occasions.
The planet hosts several other races and groups, both native and alien. They include:
*Berbils/Ro-Bear Berbils. A culture of robotic bears from the planet Ro-Bear. They have established a colony on the planet and seem to be the main providers of food for most of the humanoid races. Including the Thundercats. They have advanced technology effective combatants.
*Berserkers. A group of cyborg pirates. Vaguely resembling the Vikings. They are humanoid but their origin was never explained. Members included Cruncher, Hammerhand, Ram-Bam, and Top-Spinner. An epidode depicted Hammerhand as a ghost needing a host body. This was ignored in his later appearances.
*The Bolkin/Bolken and Wollos. Two similar races of anthropomorphic animals, seemingly humanoid sheep. The Bolkin/Bolken have been depicted as shepherds and farmers. The Wollos as farmers, carpenters and fishermen. They do not have particularly advanced technology, depicted as your average Medieval peasants.. They are gentle, pacifistic and more-often-than-not victims of raiders.
*Brute-Men. A race of anthropomorphic bovines. Primitive in culture and barely sentient. But very strong. Used as slaves by the Mutants.
*Cavemen. Another primitive race. Human but not modern-looking ones. They resemble the stereotypical Neanderthal of fiction. Very strong, rather brutal, barely able to communicate with others.
*Crabmen. Humanoid crabs. First seen attacking Thundercats when they entered their territory. Which is also their role in their few subsequent depictions. Might be a territorial culture. *Giants. Giant humanoids. Not particularly friendly to strangers.
*Gigantors/Giantors. More accurately Ogres, " large, cruel, monstrous and hideous". Typically armed with wooden clubs. They have been depicted as maintaining trade relations with the Trollogs, exchanging food supplies. Otherwise they are hostile to other races.
*The Living Ooze. Unclear if its part of a race or a single individual. A mass of talking ooze, inhabiting an area next to the River of Despair. Sentient and motivated in destroying everything green, clear, bright, and white. *Lunataks. A group of intergalactic raiders, originating from the Moons of Plun-Darr. Apparently a distinct species from the Mutants and very different in appearance from each other. They invaded Third Earth a long time before, perhaps centuries prior to the arrival of the Thundercats. They were defeated by Mumm-Ra and encased them in molten lava. He later revived them in an attempt to use them against the Thundercats. Their members include Alluro, Amok, Chilla, Luna, Red-Eye, and Tug Mug.
*Mermaids. A half-human, half-fish species. The only one prominently depicted was the so-called Vampire Mermaid. She lures people with her singing, theh drains them of their blood by biting their necks. She attempted to this to Tygra and failed. Uncertain if the rest of her species share her blood-drinking habits.
*Micrits. Better termed "the Little People" of mythology. Miniature humanoids, eternally fed-up with everyone stepping over them. They maintain a farming civilization. Once confiscated all the weaponry of the Thundercats just to make a point.
*Molemen. Humanoid moles. They maintain an isolated underground civilization. They were once dominated by the dictator Molemaster, a giant-sized member of their race. He was deposed.
*Mudhogs. Humanoid pigs, maintaining an underground civilization. Their regime is a monarchy. They do not appreciate trespassers. *Mutants of Plun-Darr. An alien culture who has struggled with the Thundercats for centuries. They consist of humanoid jackals, lizards, monkeys, rats, and vultures. One of their spaceships crash-landed on the planet and they ended up establishing a colony. The Mutants Ratar-O, Slythe, Jackalman, Monkian, and Vultureman have been prominently depicted. Unnamed mutants have occassionaly been depicted serving the others as henchmen. But the series was never consistent about that. *Snarfs. A sentient species native to Thundera, partly felines and partly reptiles. Loyal allies to the Thunderians. Snarf Osbert was the nursemaid of Lion-O and followed his charge on Third Earth. His nephew Snarfer later also settled on Third Earth. Other 42 Snarfs escaped the destruction of Thundera but colonized another planet ("Planet of Snarfs").
*Snowmen. A furry humanoid race, living in the frozen enviroment of Hook Mountain. They are a proud warrior race. Their most prominent member is the creatively-named Snowman, a loyal ally of the Thundercats. Also known as "Lord of the Snows". He is a chivalrous Knight, but his steed is a snowcat (a non-sentient feline).
*Tabbuts. Humanoid pigs with a horn growing in their foreheads. They are greedy and money-obsessed. Often seen as traders, landlords to the communities of Bolken and Wollos, and targets for bandits. They will do everything for money. But they are non-combatants.
*Thundercats. Technically a term for the upper class and warrior class of the Thunderians, an alien civilization. Once planet Thundera was destroyed the race fled in various directions. A handfull of surviving Thundercats settled on Third Earth. They include Bengali, Cheetara, Lion-O, Lynx-O, Panthro, Pumyra, Tygra, Wilykat and Wilykit. The ghosts of Grune and Jaga were also active on the planet. One as a menacing force, the other as a protector.
*Trolls. A humanoid race similar in appearance to the Bolken and Wollos. But distinct in riding giant grasshoppers as steeds and not featured as peasants. Mumm-Ra once claimed this race has a poor sense of memory.
*Trollogs. Humanoid bulldogs, cave-dwellers. Featured as bandits and raiders. Depicted as maintaining trade relations with the Gigantors/Giantors. Exchanging food supplies.
*Tuska. Humanoid walruses, having settled in sea coasts. Their most prominent member is Turmagar, a loyal ally of the Thundercats. They use non-sentient flying lizards as steeds. They resemble dragons but are called "Gomplins".
*The Unicorn Keepers. Humanoid protectors of the (non-sentient) unicorn race. Wood dwellers. Only an elderly couple has been depicted. Unknown if there are others.