The Kzinti were a species of felinoid origin, native to the planet Kzin.
The Kzinti had a history of conflict with Earth, as the two planets fought a series of four wars between them. In what was collectively known as the Earth-Kzin Wars, Kzin was always the aggressor and Earth was always the victor. The first was fought sometime after First Contact in 2063, and the last was concluded by the end of the 2060s.
Following the signing of the Treaty of Sirius, the Kzin government, including all Kzinti territories, were essentially demilitarized. The Kzinti were forbidden to possess any sort of weapons or starships - except police vessels, which were to be used solely for peacekeeping.
In 2269, Dr. Keniclius 5 cited the Kzinti's ongoing rivalry with Earth as a reason for the necessity of eugenics experiments to improve the human breed.
Later that year, Kzinti archaeologists discovered two Slaver stasis boxes on Kzin. They turned one over to Starfleet to have transferred to Starbase 25; the second they kept for themselves, only to discover that it was empty.
Based on various lucrative finds in previously discovered stasis boxes, the Kzinti hoped to find a potentially powerful weapon, which they could use to defeat the Federation. Secretly supported by the Highest of Kzin, some privateers acquired a "stolen" police vessel to conduct their mission. If they were successful in their theft of the other stasis box, they would be heroes; if they failed, they would be repudiated by the Kzin government.
On stardate 4187.3, a group of Kzinti privateers lured the long-range shuttle Copernicus into the Beta Lyrae system, using the empty stasis box. They then captured the crew of the Copernicus on the surface of a barren ice planet, placing them in a police web aboard their privateer vessel, the Traitor's Claw.
Upon inspection of the stolen stasis box, the Kzinti discovered a Slaver weapon, whose functions they immediately began to test. By the time they reached the last function, the device, a sort of "smart computer", interpreted the Kzinti as enemies and self-destructed, killing three of the Kzinti privateers and severely damaging their vessel.
The Kzinti were large, carnivorous, cat-like beings that stood over two meters tall. Their anatomy was extremely well reinforced, as they possessed multiple hearts, and vertical braces along their ribs.
Some male Kzinti were capable of reading minds; however, psychologically, they were often unhappy and neurotic. Although there were no sure ways to prevent a Kzinti telepath from reading someone's mind, it was found that mental imagery of eating vegetation dissuaded them from making such attempts.
The Kzinti were a prejudiced, war-like species who often spoke of pride and honor. Males had no respect for males of herbivorous species, such as Vulcans, or of females of any species. This lack of respect stemmed from the fact that on their homeworld, Kzinti females were simply "dumb animals".
If a Kzin was defeated in battle but allowed to live, that action was considered the ultimate insult. In such a case, the Kzin had to seek revenge by challenging the attacker to a duel before the attacked Kzin was allowed to seek either medical assistance or reinforcements.