Kzinti

Tall, cat-like, and violent. Kzinti are up to three meters tall and covered with orange fur, often with dark spots or stripes. They have naked, ratlike tails and folding "chinese-parosol" ears. Their Empire, headed by the Patriarch, is larger than mankind's, but after several wars the balance of power has shifted in favor of humanity. Kzinti tend to attack before they're ready, and multiple ill-advised incursions into human space have killed off the most aggressive and left the race as a whole slightly more cautious and easy to deal with. Female kzinti are nonsentient, having been bred that way centuries ago. When a kzin bares his teeth it's a sign of aggression.

Read more about the Kzinti here.

Pierson's Puppeteers

The first puppeteer was encountered by a spacer named Pierson, who described it to his companions as "a three-legged centaur with two Cecil the Seasick Sea Serpent puppets on its hands, and no head," or something like that. These unlikely beings are an old race, noted mainly for their business acumen and their inborn cowardice. The Puppeteer company General Products produces a line of invulnerable starship hulls, and these and other technological wonders have allowed the Puppeteers to gain major footholds in the economies of all the important races in Known Space.

Puppeteers are cowards, and indeed the race views cowardace as noble. The only puppeteers you'll meet in Known Space are therefore mad--only the insane ones board spaceships.

When they heard of the Core Explosion, the puppeteers packed up their planet and moved it, along with four agricultural worlds. The five planets of the "fleet of worlds" travel in a rosette, orbiting one another, powered by the heat of close to a trillion puppeteers and lit by artificial suns. Where they are heading remains a mystery, although some people have guessed the truth.

Read more about the puppeteers here.

Outsiders

Evolved in the cold of space, Outsiders are dealers in information. Their technology is very advanced, and from time to time they sell it to other races. They resemble thick-handled cat-o'nine-tails; their starships are vast, fragile-looking arangements of latticework exposed to space.

Outsiders follow Star Seeds, which are giant seed pods that migrate from the core of the galaxy to the rim and back. No one knows why they do this, but it is known that the Puppeteers used a device called a Star Seed Lure to send the Outsiders through human space in time to sell us the secret of hyperspace before we lost our first war with the kzinti.

Grogs

A sessile species found on the former Kzinti-ruled planet known as Down, Grogs are furry cones that would be almost cute except for the theory that they are descended from Thrints. Grogs claim to have never heard of Thrints, and the theory is more or less impossible to test.

Since adult Grogs don't move, they use their telepathic abilities to guide food animals into range of their grasping tongues. The extent of their telepathy can't accurately be tested, because no one trusts the results. But Grogs act as herders for the settlers on Down and have managed to attain standing in Known Space by disassociating themselves from Slavers as much as possible.

Bandersnatch

Giant slug-like beasts the size of blue whales. Found on Jinx and scattered other planets on the edges of Known Space, the Bandersnatch are in fact sentient. They were built by the Tnuctipun as a food animal for the Thrints, but since Bandersnatch are immune to the Power, they made perfect spies. They survived the war that killed the Thrints and all the other races at the time, and lived on in solitude until humans and other races populated the galaxy again. Nowadays they trade with other species by granting hunting rights.

Martians (extinct)

The mysterious martian culture under the dusty surface of Mars has never been fully explored. A primitive culture, they used spears to slash the life-support bubbles of the few human expeditions to the red planet. They have a very exotic chemistry - they explode on contact with water. They were wiped out when a water-ice asteroid impacted on the surface of Mars.

Thrintun, Tnuctipun (extinct)

1.5 billion years ago, a race called the Thrintun ruled the galaxy through telepathic Power. They were also known as Slavers. The Thrintun were rather stupid, and they were very dependant on their slaves. One slave race, the Tnuctipun, were allowed certain freedoms because of their skill with biogenetic engineering, and other technology. They used that freedom and their skills to wage war against their oppressors. When the war was over, no creature with a notochord survived. The Grogs are thought to be descendants of the Thrintun.

Jotoki

These are a Kzinti slave species, rarely encountered by humans. They are symbiotic, joining together into units of five to form a starfish-like sentient being. Rumor has it the Jotoki had their own space empire before they made contact with the Kzin, who were still using swords and crossbows at the time. They hired the Kzin as mercenaries, only to find themselves enslaved by their much-more-savage employees. This is more-or-less impossible to confirm, as the only Jotoki one meets these days are menial slaves of the Kzinti.

Pak

Evolved on a world much closer to the galactic core, the Pak have three stages of life: infant, breeder, and protector. Infant is self-explanatory; breeder is what we know as homo Habilis. Protector is the next stage of life, attained with the help of a virus called "tree-of-life," which gives the Pak long life, extra brainpower, thicker skin and an overwhelming urge to protect its own bloodline at the expense of all others.

A colony of Pak landed on Earth millions of years ago, but tree-of-life failed to grow properly and the Protectors died out, leaving Homo Habilis to develop into Homo Sapiens. The Pak still exist in other parts of the galaxy. They are very warlike, with each protector striving to protect its bloodline above all others.

Kdatlyno

Looking like demons from a children's book, the leathery-skinned Kdatlyno were a Kzinti slave species before they were freed in the Man-Kzin Wars. A proud race, they create art and other artifacts for sale in Known Space.

Kdatlyno have no eyes; they "see" by a form of sonar. Kdatlyno touch-sculpting is a unique art form, meant to be experienced with the hands more than the eyes.

Trinoc

This three-eyed race from beyond Known Space has a culture based on gambling. This explains how they can be so paranoid and yet still have the courage to explore the galaxy -- the odds seem to support exploration.

Dolphins

Earth's cetaceans were declared sentient during the early days of colonization. They are great companions and notorious tricksters, and are employed as sea-farmers on any human world that has a livable ocean.

Pierin

No description of this race is given in any story, but the Ringworld Companion, a supplement to Chaosium's Ringworld Role-Playing Game, suggests this is a Kzinti slave race of nosy, over-friendly avians from a light-gravity world who resemble great horned birds at a distance, but at closer view seem like occult idols brought to life. The raspy screeches and atonal clicks of their speech make them difficult to understand.

Pleione - Mentioned briefly in the hard-to-find story "The Color of Sunfire." No specific information is given.