Machaeroides

Machaeroides ("dagger-like") was a genus of sabre-toothed creodonts which lived during the Eocene (56 to 34 mya). Its fossils were found in the U.S. state of Wyoming. It is the earliest known sabre-toothed mammal. 
It looked like a small, dog-sized saber-toothed cat. "Machaeroides" had a longer skull than a cat-family animal, and it walked plantigrade, with its palms and heels on the ground, instead of on its toes like a cat or dog.
The long survival of these small animals shows that their killing method suited small cats as well as large cats.