The sexual dimorphism in the colors of Hippowdon does not have its origin in a mating display akin to Unfezant's; Hippowdon find the opposite sex's color, whether tan or black, as drab as humans do. Both sexes of Hippowdon are colored for camouflage, and their colors mirror those of the sand they breathe.
The tan sand of male Hippowdon is that of most deserts, and in those deserts male Hippowdon can easily sneak up on prey, hidden within a storm of their own breath that is mistaken for a piece of desert picked up by the wind. Unlike the Hippopotas, the large and lumbering Hippowdon require this camouflage to survive, so male Hippopotas will usually return to the desert of their fathers once they sense that evolution is soon.
The black sand of female Hippowdon, on the other hand, requires them to live near volcanoes. Volcanic sand is as black in color as they are, but volcanoes which produce enough sand are not easily found near deserts. This serves as the main break on Hippowdon population growth, for although male Hippowdon can easily survive in places from the Sahara to Orre, females are too easily seen for them to find food. Conversely, female Hippowdon have little difficulty on many volcanic islands and beaches, but the males are equally visible.
Hippowdon do best in that intersection of volcano and desert where most of their prey inhabit, and their unusual sexual dimorphism confines them to that biome in the wild, at least to a degree. But if a volcano or desert alone has sufficiently abundant prey, vast hordes of Hippowdon will migrate there, summoned by the opposite sex, and create enough black or tan sand that both can call it home.