In addition to its abundance in the middle Eocene of the Western Interior of North America, fossils of the brontothere Metarhinus are known from similar aged deposits in Southern California. Because of additional material from the Friars and Santiago Formations, San Diego County, California, M. pater Stock (1937), a dubious species formerly known from a single specimen, can now be differentiated from other nominal species of Metarhinus, M. fluviatilis Osborn (1908), and M. abbotti (Riggs, 1912). Inclusion of M. pater into an analysis of brontotheriid phytogeny establishes Metarhinus as a monophyletic genus of uncertain origin. The three species of Metarhinus differ in the shapes of their nasals and the lengths of their nasal incisions, whereas other aspects of their skulls are undifferentiated. The majority of Metarhinus specimens lack species-diagnostic features and thus have unknown specific identities. At least two species co-occur wherever Metarhinus is abundant. We question whether these morphospecies represent population-lineage units (i.e., species), or if they are intraspecific variants possibly explained by ontogeny or sexual dimorphism. Dental wear stages, used as an age proxy, are uncorrelated with Metarhinus morphospecies. Likewise, variation observed within Metarhinus samples is inconsistent with sexual dimorphism. Coefficients of variation (CVs) suggest that the cheektooth dimensions of Metarhinus assemblages are more variable than monospecific referent samples, and that most assemblages are mixtures of two size-differentiated species. The persistent sympatry of Metarhinus species is a biogeographic pattern that appears to be atypical in comparison to other brontotheriids and could indicate an unusual case of sympatric speciation.