Amphicyon, the bear-dog, was probably the largest land-dwelling predator of its day, about the size of a modern grizzly bear. It was really neither a bear nor a dog, but a member of a now-extinct family of carnivores that, in evolutionary terms, is between bears and dogs but has no living relatives. Its name means "ambiguous dog." Amphicyon's bone structure is heavy, with massive thigh and shoulder bones to support strong muscles.

Amphicyon was built like a lion, and probably ambushed its prey with short, powerful bursts of speed, just as lions do today. Bear-dog prints that were left some 14 million years ago show a stride length about equal to the animal's body length, minus the tail. They also show that the bear-dog was pacing—moving the two left legs and the two right legs alternately, the way bears do today, though at a much slower stride.

Although this scene looks grim for the little antelope, in evolutionary terms the animals are actually going in opposite directions. Fourteen million years ago the bear-dog was on the verge of extinction, while the Ramoceros (pronghorn antelope) was near the beginning of its history.