Platyhystrix may have been preyed upon by larger temnospondyls such as Eryops, or by larger carnivorous reptiles, which were becoming more common and diverse in the drier climate of the Permian. The skull was large and strongly built, with a frog-like face. Platyhystrix had a compact body, reaching 1 metre (3.3 ft) long including the tail, and its short, sturdy legs indicate a mainly terrestrial life.
Platyhystrix appeared rather unusual: the dorsal vertebrae were extraordinarily lengthened, and in life they probably formed a skin-covered sail. This structure was possibly for thermal regulation, as in other animals of similar appearance, such as the pelycosaurs Dimetrodon and Edaphosaurus. The back of Platyhystrix was also covered with thick hard plates, similar to those of its near relative, Cacops.