Tnuctipun were small, arboreal pack predators, averaging about 3–4 feet long. Their heads were long and lean, and their eyes opened laterally. They were highly intelligent (average IQ estimated around 130–140, and social. As befits their carnivorous nature, they were also ruthless, aggressive, and cunning. Their word for alien most closely translates to "food that talks".
Two billion years before humans evolved, the galaxy was ruled by the Thrintun, who telepathically enslaved other species, including the Tnuctipun.
The Tnuctipun invented most of the technologies from that era, including stage trees (trees containing solid rocket fuel in their trunks, originally used as cheap rocket boosters, now evolved to seed themselves across star systems), sunflowers (flowers with integral parabolic mirrors that can focus sunlight to deadly effect), and stasis fields (a time dilation device). They were also known to have direct conversion of mass to energy and a telepathy shield (these two technologies are lost by the time most Niven stories take place). In order to think creatively, the Thrintun allowed the Tnuctipun some freedom. The Tnuctipun used that freedom to stage a rebellion against their masters, the culmination of a carefully thought out, centuries-long plan.
The true nature of some of their inventions became clear: Bandersnatchi, thought to be non-sentient livestock, were in fact intelligent, created as spies immune to telepathy. Sunflowers turned against their masters and burned Thrint homes to the ground. Other Tnuctip inventions were designed to shape Thrint society to weaken it. Sunflowers encouraged a trend for the slavers to live in isolated manors, surrounded by slaves. Mutated racing viprin (fast-running creatures raced for entertainment and gambling) ruined the existing viprin herding business, which along with other similar inventions led to an economic depression prior to the Tnuctip revolt.
The war escalated until the Thrintun, rather than accept defeat, employed a device that amplified the sphere of influence of a Thrint's mind control to encompass the entire galaxy. And they gave a simple command: Die. And everything in the galaxy that had evolved a backbone perished, including any Thrintun not protected by a stasis field.
Possibly responsible for the creation of the Pak Protectors.