In a certain solar system in the Small Magellenic Cloud Galaxy remarkable things are happening. Slimy creatures have crawled from the oceans, grown lungs, scales, and later fur. One small lithe creature is beginning to form a successful specied. It will continue to grow and thrive and eventually become the dominant species. This is the ancestor of ancient Thunderians. This proto-feline splits into different species, all fierce carnivores. The climate shifts, becomes hotter, harsher, and one of these species takes to the trees to avoid the rising water. It slowly begins to eat plants and slower still develops thumbs. It becomes bipedal to look far over the trees for prey... and predators.
For giant beasts have developed, like terrestial dinosaurs only in their size and ferocity. On this planet, the Age of Reptiles was short and the Age of Mammals cam quickly, too quickly for some to change. One, an ancestor to the proto-felines, quickly develops into a short, stocky creature, intelligent and fierce. These are the Snarfs. They begin to develop language and coax the felines out of the trees. The felines begin to learn language and create tools.

Many hundreds of thousands of years later, the felines- now recognizable as Thunderians, although digitgrade and tailed, have built cities and rockets. The Snarfs live seperately with their own civilizations- not cities, but communes in the forest with nature.
The Thunderians are wise enough to quickly develop ways to not damage nature, but they bend it sharply to their needs. They level mountains and divert rivers for their own conveience and jump into space- eager to find other feline species.

Instead they find humans- or close enought not to matter. It is not clear whether these are, in fact, terrestial homo sapiens or some other species like them- perhaps ancient Atlanteans or Lemurians for Earth- still First Earth is primitive. In any case, these humans act terrestial- they shun and enslave the Thunderians and so they retreat quickly.

They cower in their planetary prison, afraid of the humans but wanting to join the universal community. The Thunderians have only simple sublight travel with suspension capsules- their magic is forgotten, although the Snarfs retain books on it.
Then one year, perhaps 100 years after they were forced back to their own planet, own solar system, and having fought and conquered and divided into clans- white stripes, and spots, and spots and stripes, and orange, and blue- one Thunderian, a white tyger, discovers the secret of genetics and how to manipulate them. This is an astounding discovery and is soon followed by the discovery of antimatter and the development of FTL - faster than light travel. The Thunderians quickly begin to experiment and the Snarfs pull farther away, for they disapprove. "This nattering and explosions and things," says one, the founder of Snarf College- for war and competition has been replaced by learning and cooking and snarfball- "Well, it'll never come to anything good. They should just settle down, like us, and relax. Got to get it out of their systems, I suppose, like we did. I just hope they don't take us and the planet with them. Snarf! Shnarf!" A valid worry indeed.
The Thunderians try to conform themselves to human standards and set off to join the universal community - and are rejected again. This time they give up- they conquer and colonize and pillage. In the process they destroy a good much of the planet - and magic is rediscovered.

Alchemy becomes a great art- a gentleman's art- and alchemist like Malcar conquer and rule until they too are put down, killed, burned, and sealed forever in Tomb Town.
The climate shifts horrendously.
A vast ice age falls. Blame is thrown this way and that, but it is just Nature's way of saying that she is still mightier. The Thunderians emerge stronger and changed- no longer human, but not old Thunderian either. They are stocky, with long hair and a vast array of patterns. Other things emerge- the mutants- animals altered in an attempt to create a community, but enraged at their creators. They escape, survive, conquer, then leave, taking the last of the spaceships with them. One last thing emerges- the Thundercats with the Sword of Omens. Palar, a great and wise sorceror created two swords. The first was captured and altered by Krondor, formerly his apprentice, a reptilian pretending to be kind and true. This Sword became the Sword of Plundarr. The second became the Sword of Omens, The Sword of Plundarr's equal- for all of Palar's soul and skill went into those swords, but they became very different. The Sword of Plundarr was a sword of fire and destruction: the Sword of Omens, a sword of warning and justice. And there was no other stone like the Eye of Thundera, which seems to have no origin.

The mutants returned and took Thundera's moons in hopes of destroying it - but the Eye of Thundera restabilized the environment and spread the Thundercat nobility into all Thunderians. With the mutants went the last of the human Thunderians- these became the Luna-taks.
And so the mutants, the Thundercats, and the Luna-taks battled until, finally, Jaga the Wise brought peace by sealing up the Sword of Plundarr.