The taxonomy of the American Onthophagus Latreille, 1802 included in the “hirculus” group is revised, and a study of their morphology allows for the delineation of five species-complexes. Herein, we provide a diagnosis of the “hirculus” group, an illustrated key to the complexes, along with a descriptive overview of their taxonomy and geographic distribution. Onthophagus hirculus Mannerheim, 1829 is considered to be a junior synonym of O. hircus Billberg, 1815, which is here demonstrated to be a valid species name. Thus, the “hirculus” group is here renamed the “hircus” group. Furthermore, a general scheme of the genital organs of American Onthophagus is provided, including names of different anatomical parts and a brief discussion on the need for nomenclatural stability for the genitalia of scarab beetles.