Dung beetles in the genus Onthophagus Latreille, 1802 develop horns from their pronotum or heads. The size of the horns is related to body size and follows either a linear or a distinct, polyphenic relationship. Here, we report the relationship between body size and pronotal horn size in Onthophagus hecate (Panzer, 1794), a common species native to North America. Like other species, the size relationship between pronotal width and pronotal horn was linear. For males, a longer pronotal horn is associated with more furcation and an increase in protuberances: beetles with pronotal horns < 3.65 mm had 2 protuberances, and beetles with horns > 4.27 mm had 4 protuberances. Males also displayed a prominent clypeus, which like their pronotal horn, increased linearly relative to their body size. These allometric relationships provide a baseline for further studies using O. hecate as a model for understanding the genetic and physiological mechanisms of developmental plasticity in insects.