It was collected during a 1966–1967 expedition but not named and described until recently. Paul Sereno, who had recognized it as unusual in the 1980s, published a description in 2012. The type species is P. africana. Pegomastax differed from other heterodontosaurs by details of the skull. The lower jaw was robust, with a short beak. Like most other heterodontosaurs, Pegomastax had an enlarged canine-like tooth at the beginning of the lower jaw's tooth row, which may have been used for defence. The rest of its teeth were those of a herbivore.