Kzinti (singular kzin) are large and very aggressive felinoid aliens.
Kzinti evolved from a plains hunting felid on a slightly colder, drier planet than Earth. The Kzin word for their home planet translates as Homeworld. The world is often known as Kzinhome by the Kzinti themselves. The Kzin home world is the third planet orbiting the star 61 Ursae Majoris.
The Kzin civilization was at an iron-age technological level when an alien race called the Jotok landed and made stealthy First Contact with a tribe of primitive hunter/gatherer Kzinti. The Jotok were interstellar merchants looking for a species they could use as mercenaries.
Once the Jotok had taught the Kzinti how to use high-technology weapons and other devices (including spacecraft), the Kzin rebelled and made their former masters into slaves, as well as the occasional meal. The crest of the Riit (Royal) family appears to be a bite mark, but is in fact a dentate leaf, with the words "From mercenary to master." written around it in Kzinti script. Kzinti overran an enslaved many races and cultures, not only the Jotoki but also the Kdatlyno, Pierin and Whrloo. The Chunquen were exterminated.
Kzin society is extremely male dominated. The leader of the race is called the Patriarch, which is a hereditary title. The Kzinti call themselves "Heroes" or the "Heroes Race" and because they believe themselves to be "heroes," their society places a very high value on "acting Heroic" and behaving in a heroic fashion.
To Kzin society, "heroic" means being honorable and having integrity. Kzin honor, called strakh, is similar in many ways to the samurai code of Bushido. For example, if the Patriarch gets meat from a sellers' market stand, the seller gains considerable strakh which will bring honour to the seller allowing him to get better customers - which leads to more strakh, which gives the seller a higher status within the community.
Once Kzinti gained access to genetic-manipulation technology, they started manipulating themselves in order to bring out the most "heroic" qualities and recede undesired ones. To this end, because females are not valued except as bearers of children, the male-dominated Kzin society bred (most of) their own females into sub-sapience.
Kzinti are often described as anthropomorphic tigers, but there are significant and visible differences. Kzinti are larger than humans, standing around 8 feet (2.4 m) tall and weighing around 500 pounds (230 kg). These tiger-sized bipeds have large membranous ears, a barrel-chested torso with a flexible spine, and large fangs and claws. One human gave an apt description of Kzin as "eight feet of death."
Unlike some popularly depicted anthropomorphic animals, Kzinti stand on two legs like humans do; they do not have digitigrade or "backward-bending" legs. Their hands end in three fingers and an opposable thumb, all with retractable claws.
They are covered with a thick coat of long fur that comes in various combinations of orange, yellow, and black. Full black coats are rare.
Their tails are naked and are similar in appearance to a rat's tail, and their noses are black. Kzinti ears have fur only on the outside of the ear and only about half-way up the ear itself, usually appear pink, and are shaped liked a segment of a Chinese parasol (or cocktail umbrella; they are also sometimes described as "bat-winged"); they can fold back flat against the head for protection during a fight.
Female (kzinrett) main-culture Kzinti are not sapient, although among the archaic Kzinti found on the Ringworld some are. The Kzinti in Known Space have bred intelligence out of their females. This is due to the work of the Black Priests, a powerful cult within the Patriarchy whose members all have completely black pelts, have been responsible for the breeding program to isolate the telepath gene and preserve kzinrret subsapience. Kits are tested while young; a female who displays too much intelligence is taken away and killed, and a male who displays knowledge they could not have gotten except by telepathy is taken away and addicted to sthondat lymph to become a Telepath. On Kzinhome, however, the czrav and forest nomad prides, outcasts from mainstream Kzinti culture, never accepted the Black Priests, and in consequence have fully sentient kzinrretti, although this fact is a carefully held secret.
Kzinti speak in a hissing language called the Hero's Tongue, which in its written form resembles commas and periods.
Humans fought several brutal interstellar wars with the Kzinti Patriarchy. Kzinti tactics are somewhat cat-like in nature, "scream and leap" being the primary mode of declaring a challenge.
The first Man-Kzin War ended when the humans obtained the faster-than-light drive (FTL) from the Outsiders, after which the Kzinti ships stood no chance against the FTL warships. Throughout the rest of the wars with man, the Kzinti tended to always attack before they were ready, and subsequently lost all of them. As a result of this most of their empire was lost; in the peace treaties that resulted, colonies and slave planets were ceded to man or given independence.
In Ringworld it is revealed that this was in part due to clandestine meddling by the Pierson's Puppeteers. They saw in the aggressive Kzinti a major threat, and orchestrated the events that led to the humans getting FTL ships and thereby ensuring the human victory of the first war. Each of these Kzinti defeats eliminated the most aggressive individuals from the Kzinti gene pool and thus made the Kzinti more "manageable" from a Puppeteer point of view. By the time Ringworld takes place, Kzinti are able to deal with other races diplomatically, rather than by attacking and enslaving them.