The Kzin (plural Kzinti) are a very warlike and bloodthirsty race of Felinoid aliens in Larry Niven's Known Space series. (They also appeared in The Slaver Weapon, an episode of Star Trek: The Animated Series.)
Kzinti are thought to have influenced the creation of the similarily Felinoid Kilrathi, the primary antagonists of the popular Wing Commander video game series of early-to-mid-nineties. Whether this is true or not, a small part of Wing Commander II takes place in a region of space called the Niven Sector.
Warning: Wikipedia contains spoilers
Kzin are evolved from a plains hunting cat, stole their current space-faring technology, and bred (most) of their own females into sub-sapience. They are larger than most humans, they have large membrane ears, a cylndrical torso without a spine, and large fangs and claws. A small percentage of Kzinti are stunted and exploited addictive telepaths, and they aren't legally allowed to breed.
They are the first on-going alien contact that humanity has within the KS universe. The onset of their relationship with humanity ends the golden era, where humanity had almost completely succeeded in re-writing history into a non-violent whitewash, and where organized violence was only known about by roughly 1 in 10 people, and there was no interpersonal violence, except occasional out-bursts in the belt where medical and psychological care were spread too thinly.
The Kzinti contact a laser-driven exploration ship, with vast technical superiority, gravity planers, telepathy, and a vast empire. They attempt to kill the human crew in a flashy manner when the telepath learns that they don't even understand the concept of weapons, and fail.
In several different stories by other authors playing in the universe we see a number of Man-Kzin wars take place.
Eventually we learn that the Kzinti were deliberately led to encounter humanity by the Puppeteers. The puppeteers hoped that the culling of more than 1/4-1/3 of the aggressive members of the Kzin race with every war would result in a more peaceful race.