Pachyaena (literally, "thick hyena") was a genus of heavily built, relatively short-legged mesonychids, early Cenozoic mammals that evolved before the origin of either modern hoofed animals or carnivores, and combined characteristics similar to both.

However, a 2007 paper suggests that Pachyaena may be paraphyletic, with P. ossifraga being closer to Synoplotherium, Harpagolestes and Mesonyx than to P.

[2] Unlike many mesonychids, Pachyaena is known from skeletal material in addition to skulls and jaws.

Analysis of three wolf- to bear-sized species from the early Eocene of Wyoming (Willwood formation) indicates they all had many adaptations for running, including paraxonic compressed feet with a vestigial first digit, lower sections of the limbs elongated compared with the upper sections, and limb joints with movement mostly restricted to a sagittal plane (back-and-forth movement).

Pachyaena was likely built for endurance rather than speed; the overall body shape of the genus would have resembled a modern tapir.