Size and Weight:
Although it wasn't quite as mammal-like as its close cousin, Cynognathus, Thrinaxodon was still startlingly "modern" by early Triassic standards. Paleontologists believe this cynodont (a subgroup of the therapsids, or mammal-like reptiles, that preceded the dinosaurs and eventually evolved into the first true mammals) may have been covered in fur, and also may have had a moist, cat-like nose. Completing the resemblance to modern tabbies, it's possible that Thrinaxodon sported whiskers as well (and for all we know, orange and black stripes).
What paleontologists can say for sure is that Thrinaxodon was among the first creatures whose body was divided into "lumbar" and "thoracic" segments (an important anatomical development, evolution-wise), and that it probably breathed with the aid of a diaphragm, yet another feature that didn't come fully into mammalian vogue until tens of millions of years later. (Recently, a Thrinaxodon specimen was discovered curled up alongside the remains of the prehistoric amphibian Broomistega; apparently, the latter creature crawled into the burrow to recover from its wounds, and both occupants drowned in a flash flood.)