Hyaenodon
500 kilograms in weight, 1.4 metres at the shoulder, and around 3 metres long. Of the extinct order of the Creodont mammals, which had once fulfilled the same role as the Carnivora - the modern mammal predators such as the Felines, Canines and Ursidae.
They lived from 45-20 Million Years Ago, ranging from Labrador sized hunters to rhinoceros sized apex killers. During the Oligocene Epoch 25 Million Years Ago, the larger form of Hyaenodon (Hyaenodon gigas) would fight with the roughly equally nasty Entelodon - they seemed to be arch-rivals.They battled for 5 million years. Hyaenodon and the remaining creodonts survived for millions of years, but they were eventually pushed to extinction by the more advanced Carnivorans, which includes modern day cats, dogs and bears, as well as extinct predators like the famous Smilodon.