On the cartoon Thundercats they arrive on Third Earth from planet Thundera. It seems like it's an apocolyptic Earth and shares some myths (such as King Arthur) and according to the Thundercats wiki page http://thundercats.wikia.com/wiki/Third_earth Third Earth was once known as First Earth. So why is it called Third Earth? I would assume it's the third Earth that was found but there are no other references to Earth in the series that I know of.
There are two prevalent theories:
- Mumm-Ra is using 'Earth' instead of planet. "Third Earth" is known to be the third planet in it's star system.
- or, "Third Earth" refers to their period of this Earth's history, with First and Second Earth being earlier periods - possibly coinciding with our current civilization. There is some speculation that Second Earth may have been a post-apocalyptic period between now and then.
I haven't seen it, but I hear tell of an unreleased episode that states Mumm-Ra is from First Earth. Also, there is a new series on Cartoon Network - I think - that may or may not go into more detail. I haven't seen it, so I can't say.
In The Unholy Alliance, the second episode of the series, the following dialogue is spoken:
Mumm-Ra: As long as evil exists, Mumm-Ra lives!
Slithe: What manner of being are you?
Mumm-Ra: You need only know that I am Mumm-Ra and that I know of your mission here on Third Earth. You seek to possess the Eye of Thunder.
Slithe: You know of the Eye of Thundera? Yessss?
Mumm-Ra: I have known of its power for a thousand years! From a time when this was still First Earth!
Unfortunately, in the cartoon, they never elaborate on it again.
In the the first episode, "Unholy Alliance", Mumm-Ra is shown to live in the pyramid of Giza. He also states that he has been on that planet since it was first earth. Mumm-Ra is an Egyptians who sold his sole to the evil spirits, who want him to find the eye of thundera for them. This is all explained in the series, the 85-89 series.