Three sketches of the earliest known group of saber-toothed predators, the beautifully named gorgonopsians.
These Late Permian synapsids seem to have had quite a unique way of eating. While the business end of their teeth does look somewhat like that of later actual saber-toothed cats, they practically lacked useful teeth behind the canines. That is, they had nothing to cut or chew meat with. They probably simply tore large chunks of meat off the carcass and swallowed them like a vulture or something.
The upper left animal is Inostrancevia, the largest known gorgonopsian. With a length of 3,5 meters, it was around the size class of large bears or smallish rhinoceroses, though it had proportionately short legs. It lived in what is now northwestern Russia. Rubidgea, on lower left, was almost as huge. On upper right is a particularly short-snouted Sauroctonus, a smaller animal whose fossils have been found in South Africa and the Volga region in Russia.
It's not known if gorgonopsians had bare skin, some sort of primitive bristle-like fur, scales or some combination of those. As I'm just getting familiar with their body plan, I gave them a very simple, smooth skin. They probably did have something more complex in reality, though.
Drawn after skull references in Mauricio Anton's new book Sabertooth. It's a beautiful and fascinating book. You probably should buy it.