The Tholians were an extremely xenophobic, non-humanoid race with a propensity for precision, native to the Alpha Quadrant.
In 2155 in the mirror universe, the USS Defiant had traveled back in time from 2267 in the mirror universe. When it fully clear of the interphasic rift used to bring it to this universe, it was towed by the Tholians of that universe to a small moon drydock in the Vintaak system, deep in Tholian space. There they employed several humanoids to gut the ship. (ENT: "In a Mirror, Darkly")
Jonathan Archer learned of the vessel's presence in his universe. He commandeered the ISS Enterprise and flew it deep into Tholian space while cloaked to steal the Defiant. After sending a boarding party onboard the Defiant, the Tholians spun a web around the Enterprise and destroyed it. The boarding party managed to activate the Defiant's systems, but not before the Tholians spun a tractor over the entrance of the drydock. The Defiant then destroyed three Tholian ships and the moon's drydock, before escaping the system. (ENT: "In a Mirror, Darkly")
A living Tholian was approximately the same height as a Terran. It was principally a reddish color, in varying shades, and there was the suggestion of fluid or field motion within the body. Tholians had a hard carapace that was chiefly mineral. There were six thin legs that made it possible for the being to move in any direction quickly; the legs were articulated with joints roughly corresponding to the knee and ankle of humanoid species. Each leg ended in a multi-toed foot. The legs were attached at the base of the torso and were radially symmetric. There were two arms; each had joints analogous to the humanoid elbow and wrist, and each arm ended in a multi-fingered hand. There was no neck; the term "head" did not apply. Tholians had two glowing spots near the top of their torso. They turned these to face individuals with whom they interacted, which suggested they had some sort of information-gathering organ. Tholians did have two sexes, but each individual had both sets of sexual organs. (ENT: "In a Mirror, Darkly")
Tholians existed at high temperatures – in excess of 450 Kelvin (177 °C, 350 °F). They could tolerate lower temperatures for a brief period of time; if they were exposed to too low temperatures, their carapace would crack, making low temperatures a very effective means of torturing a Tholian. This was painful or distressing; a Tholian subjected to such a temperature regime could be coerced to cooperate. In temperatures even lower, a Tholian would freeze solid and shatter. (ENT: "In a Mirror, Darkly")
Tholians evidently had something analogous to biochemical processes. Phlox from the mirror universe was able to devise a sedative; however, this compound eventually lost effectiveness. (ENT: "In a Mirror, Darkly")
Tholians were naturally able to emit various forms of radiation, which they could modulate. They could communicate over short distances in this fashion. A Tholian imprisoned aboard the ISS Enterprise in 2155 was able to summon help via this method, forcing Phlox to kill it in order to silence it. (ENT: "In a Mirror, Darkly")