The Thrix were an extinct humanoid civilization native to a planet located somewhere in the galaxy's Alpha or Beta Quadrants. Their body armor was unique and recognizable.
History
As early as 25,000 to 40,000 years ago, two Thrix visited Varda III. One died, while the other was placed in stasis by Fi'lakas.
In the 20th century, the Thrix became an extinct species.
In 2277, a landing party from the USS Enterprise including Pavel Chekov and Narva Moktar visited Varda III and rescued the living Thrix along with two dozen others in stasis. Chekov recognized the species instantly by its armor. Spock discovered the body of the dead Thrix and identified it from its skeleton.
Doctor Leonard McCoy used neuroallozine to safely revive those in stasis, commenting that the Thrix was a "living fossil" due to the race's extinction. All of the survivors were taken to Starbase 6. (TOS - Plot-Your-Own Adventure Stories novella: Distress Call!)