Onthophagus halffteri Zunino was originally described based on a single specimen (a hypertelic male), the only one known at that time. The capture of several series with an abundance of individuals allows us to now describe hypertelic (major), eutelic (medium), and hypotelic (minor) males and females. Additionally, the assignment of O. halffteri to the hippopotamus line within the chevrolati group of American species of Onthophagus Latreille is confirmed, with Onthophagus hippopotamus Harold the species that is most closely related to O. halffteri. The association of O. halffteri with nests of Geomyidae (Rodentia) is established, and the conditions that separate the distribution of O. halffteri from that of O. hippopotamus are given. Onthophagus halffteri and O. hippopotamus are two species that, while practically adjacent, never overlap in a given nest. Biogeographically, O. halffteri represents the largest eastward expansion of the hippopotamus line, which is distributed along the mountains of Mexico, particularly the Trans-Mexican Volcanic Belt.