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Several thousand species of beetles evolved impressive, exaggerated horns or horn-like structures. The horn phenotypic patterns and the developmental mechanisms are well documented especially in the Scarabaeidae, the family most predominated by species with horns. The regulation of horn expression appears to be extremely evolutionary labile to the extent that horn allometric patterns have been seen to rapidly diverge between closely related species. For this reason, it has been suggested that horn morphological pattern may be able to differentiate closely related and sibling species even when other traits fail. In this study, we used horn morphological pattern (shape and allometric variation) as a “tool” to evaluate the differentiation of two closely related scarab species, Copris klugiHarold and Copris sierrensisMatthews whose full species status has long been debated due to their high similarity. Combining traditional and geometric morphometric methods, we evidenced that male head horn phenotypic pattern is able to clearly differentiate C. klugifrom C. sierrensis, supporting the hypothesis that they are two true species.