In life, Archaeotherium probably resembled a large, fanged, peccary with bumps projecting from the side of its head. It had high shoulders, presumably to carry strong neck muscles to support the heavy head. The brain was tiny, but had relatively large olfactory lobes, suggesting that the animal had a keen sense of smell.
The largest (and type) species, A. mortoni was an aggressive, cow-sized apex predator. Rhino jaws and other mammal bones have been found with bite marks on them that match the large canines of A. mortoni. A fossil trackway in Toadstool Park depicts the path of a Subhyracodon walking forward, stopping to see an Archaeotherium approach, then breaking into a gallop with the entelodont chasing after it. In leaner times, it is suggested that Archaeotherium dug for roots and tubers, as with other pig-like mammals.
Evidence from the Wyoming Dinosaur Center suggests that Archaeotherium,like modern carnivores, kept caches of food when their hunting was unsuccessful. These bones were mainly those of Poebrotherium.