Stagonolepis robertsoni was about 3 metres (10 ft) long. It was a quadrupedal animal covered in thick armoured scales that ran down the length of the its body. A slow-moving browser, it would have used this heavy body armour to repel attacks from contemporary thecodont carnivores. Stagonolepis had a very small head for its size; it was only 25 centimetres (10 in), accounting for less than 10% of the total body length. It had no teeth in the front of its jaws, but instead had a beak-like tip that arched upwards. This would have allowed it to uproot plants in a similar manner to a modern pig. The peg-like teeth at the back of its mouth would have been suitable for chewing tough vegetation, including horsetails, ferns, and the newly evolved cycads.
Fossil remains of S. robertsoni have been found in Lossiemouth Sandstone of Scotland, while S. olenkae is known from deposits near Krasiejów, Poland. S. olenkae is stratigraphically younger the type species of Stagonolepis, S. robertsoni. The genus Aetosauroides from South America has been considered a junior synonym of Stagonolepis by some paleontologists. Two species of Aetosauroides were named, A. scagliai and A. subsulcatus. In 2002, Andrew B. Heckert and Spencer G. Lucas proposed that smaller specimens of both species belong to Stagonolepis robertsoni, and larger specimens to S. wellesi. S. wellesi itself was originally named Calyptosuchus, a stagonolepidid from the Late Triassic Dockum Group of the United States, which was considered to be a species of Stagonolepis by Murray & Long in 1989. However, most of the sequential studies conclude that both Aetosauroides and Calyptosuchus are valid and monotypic genera, the former occurs only in South America and the latter only in the United States. Stagonolepis is restricted to the Carnian stage of Scotland and Poland.