It's still debatable about humans showing up in the new series cause. Im not very sure, but who knows maybe humans will appear, because it actually turns out in the Original series (If you read the comics that continued the series after it stopped airing) Third earth is actually our earth, after Mumm-Ra took over it some Humans fled while some stayed, which is why there are natives like the warrior maidens, If you want to find out how Mumm-Ra took over it you shoul read the Comic ThunderCats: Origins Heroes and Villians, which not only shows Mumm-Ra' Origin but that he used to be human Wahankh that served the Pharoah of Ancient Egypt, if you cant find it than go to this link on the wiki, But for some reason in the new Series Mumm-Ra appears to be of Alien Origin who Landed on Third Earth, and maybe in this series Third Earth is not our Earth

Please tell me what you think with youre comments below