Archaeotherium fossils are most common in the White River Formation of the Great Plains, but they have also been found in the John Day Basin of Oregon and the Trans-Pecos area of Texas.

Archaeotherium was distinguished from most entelodonts by having an unusually long snout and large jugal flanges, extensions of the zygomatic arches that are characteristic of the group.

It was incapable of slicing meat due to its bunodont molars, and compensated for this by using its head and neck muscles together to rip off chunks of flesh.

[1] In 1920, geologist Edward Leffingwell Troxell described a new entelodont genus, Megachoerus, as part of a series of papers discussing Marsh's entelodontid collection.

[1] In 1922, William John Sinclair erected the new taxon Scaptohyus altidens, based on a partial skull, mandibles, and several bone fragments recovered from the Corral Draw locality of South Dakota in 1893 by R. E. Zuver.

[1] In 1935, Erich Maren Schlaikjer named Dinohyus (now Daeodon) minimus, based on the symphyseal region of a juvenile's lower jaw.

[15] In 1940, William Berryman Scott and Glenn Lowell Jepsen noted strong similarities between the two genera, though they stopped short of synonymising them due to the incompleteness of the latter.

[2] Henri Marie Ducrotay de Blainville is said to have suspected that the genus belonged to a carnivoran family he dubbed Subursi, based on characters now understood to be convergent.

[19] In 1955, Charles Lewis Gazin suggested that entelodonts were offshoots of Helohyinae (within a greater Dichobunidae), if not direct descendants of Helohyus proper.

Below is a reproduction of the Yu et al.. cladogram of Cetancodontamorpha:Siamotherium Hippopotamidamorpha Cetaceamorpha Erlianhyus Andrewsarchus Achaenodon Wutuhyus Proentelodon

[24] In 2007, Scott Foss instead proposed that Archaeotherium represents a late stage of a continuous North American lineage, beginning with Brachyhyops and terminating in Daeodon.

[1] Conversely, Yu et al.. (2023) recovered Archaeotherium as belonging to a polytomy with Brachyhyops and a clade consisting of Entelodon and Paraentelodon.

The type species, A. mortoni, is relatively small and slender,[16] estimated to have weighed around 150 kg (330 lb) and a skull length, measured condylobasally, (from the tip of the premaxilla to the back of the occipital condyles) of about 47 cm (19 in).

[9] The Archaeotherium specimens initially assigned to "Megachoerus" and "Pelonax" bear massively enlarged jugal flanges, and a combination of a deep jaw and knob-like mandibular tubercles, respectively.

[16] Unusually among entelodonts (with the exception of Brachyhyops), Archaeotherium's pterygoids bore a midline synarthrosis, meaning they were essentially incapable of movement.

[28] Entelodonts such as Archaeotherium had the same general tooth morphology: large incisors and canines, triangular premolars, and small, bunodont molars.

[1] The digits were unfused like those of camelids, and the toes could spread;[29] this, in conjunction with hypothetical foot pads, may have helped Archaeotherium move on soft terrain.

[25][29] Unlike the humerus, the femur was long and relatively slender, whereas the tibia was shorter and more robust; the fibula is very much reduced, though is not co-ossified.

This suggests that, as in other entelodontids, the jugal flanges and strong jaws of the genus were involved in adult social interactions over obtaining and processing food.

Thus, it can be reasonably assumed that Archaeotherium's jugals supported large preorbital glands used for chemical communication, signalling readiness for mating.

[19] Healed bite-marks on the frontals, lacrimals and maxillae, as well as an A. scotti specimen with a damaged left cheek flange, suggest that at least some Archaeotherium populations engaged in agonistic facial biting.

[19][31] In such confrontations, one animal may have attempted to fit the head of the other in its mouth and bite down with the canines and incisors, similar to modern dromedary camels.

[33] The type species, A. mortoni, bore specialisations for biting and chewing resistant objects, such as hard fruits, stems, and bones.

Tooth wear patterns suggest that its front teeth were often used to strip leaves from plants, though there is a lack of soil scratches that would indicate rooting in the ground.

Based on palaeosol analysis, the lower levels of the White River Formation represent a forested environment with a high water table, dominated by taproot-bearing dicotyledons.

Examples were maples (Acer), plane trees (Platanus), and oaks, (Quercus), along with an extinct member of the avocado family (Cinnamomophyllum).

Smaller representatives of the late John Day Formation's fauna were soapberries (Dipteronia), roses (Rosa), and the evergreen shrub genus Mahonia.

[41] The Orella Member of the Brule Formation, a subunit of the White River Group, bears the leptictid Leptictis, the aforementioned Hyaenodon, the nimravids Dinictis and Hoplophoneus, the amphicyonid Daphoenus, the canid Hesperocyon, the perissodactyls Hyracodon and Mesohippus (plus an indeterminate rhinocerotoid), the merycoidodonts Merycoidodon and Miniochoerus, the camelid Poebrotherium, the leptochoerid Stibartus, the hypertragulid Hypertragulus, the leptomerycid Leptomeryx, and the rodents Ischyromys and Paradjidaumo.

[41] The Lower Blue Basin section of the John Day Formation preserves the metatherian Herpetotherium, the nimravids Dinictis and Hoplophoneus, the amphicyonid Temnocyon, the canids Archaeocyon, Enhydrocyon and Phlaocyon, the equids Mesohippus and Miohippus, the rhinocerotid Diceratherium, the tayassuids Perchoerus and Thinohyus, the merycoidodont Eporeodon, the agriochoerid Agriochoerus, the hypertragulid Hypertragulus, and the rodent Haplomys (plus an indeterminate eomyid).

[46] The Turtle Butte Formation, from which the latest species (A. trippensis) is known, preserves the canids Enhydrocyon and Leptocyon, the nimravid Hoplophoneus, the equid Archaeohippus, the merycoidodonts Megoreodon and Paramerychyus, and the camelid "Protomeryx" (Miotylopus) leonardi.