Currently only the single species Patagorhacos terrificus is known, which is represented by two highly fragmentary specimen, one belonging to the back of the skull and the other being the distal end of a leg bone.

Fossil remains of this phorusrhacid have been discovered in Paso Córdoba, a Natural Protection area in the Rio Negro Province of Argentina.

The rocks of the area belong to the Chichinales Formation, which dates to the early Miocene (Colhuehuapian in accordance with South American land mammal age (SALMA) classification.

Despite the lack in overlap, the ulna was assigned to Patagorhacos for its presence in the same locality and strata as the holotype quadrate and the fact that its size is consistent with the type specimen.

The local fauna includes astrapotheres, the litoptern Cramauchenia, the notoungulates Colpodon (a leontiniid) and Cochilius volvens (an interatheriid), rodents (Willidewu esteparius and Australoprocta), xenarthrans and borhyaenoids.