They were a cold-blooded, 2-meter-tall reptilian race, and loosely resembled giant snakes that had evolved arms and legs. They could comport themselves as bipeds or quadrupeds while retaining a serpentine flexibility of the lower body.
They were carnivorous and fed on living prey, paralyzing it with a curare-like venom injected through their fangs before swallowing it whole. They possessed the snake-like ability to dislocate their jaws, which enabled them to swallow a human whole.
However, they were deaf and instead used sight, scent and sensitivity to ground-born vibrations to perceive the world.
Their society was purely hunter-gatherer based, and with the development of interplanetary travel they established other planets as hunting grounds.
The Kan'ess and the Federation encountered each other in the year 2262, via the intermediary of Captain Christopher Pike, on the final year of a five-year voyage after the incident on Talos IV. While on a routine investigation of an uninhabited world, Captain Pike discovered the hard way that it was a Kan'ess hunting world when a party of the reptilians descended to the planet and began hunting the Starfleet scouting party. Four of Captain Pike's crew - Brandt, Norgay, Chisholm and D'zekeo - were devoured by the hunters, and Captain Pike allowed himself to be taken captive, in an effort to give Spock a chance to escape the planet and warn the rest of the crew aboard the USS Enterprise.
Captain Pike subsequently escaped from the Kan'ess homeworld after spending several days in their captivity, foiling an effort by them to lure the Enterprise to the planet's surface so that they could devour the crew, having developed a taste for human flesh, and strip the Enterprise down in order to advance their own interstellar technology.
The precise result of the Kan'ess first contact with humanity is unknown. After Captain Pike gave his report, the Federation began drafting a directive to cordon off that sector of space until the damage that Captain Pike's unintentional presentation of both the existence of other races and an entirely new method of communicating had been ascertained. (TOS novel: Burning Dreams)