https://en.m.wikipedia.org/wiki/Inostrancevia
Inostrancevia was a type of synapsid called a gorgonopsian—a group of saber-toothed beasts. It was about the size of a tiger but vaguely reptilian in appearance, with leathery skin like a rhinoceros.
Sounds like it includes mammals in the cladistic sense though. So pre-mammals?