Heterophallus is a genus of livebearing fish in family Poeciliidae, closely related to Gambusia and endemic to river systems of southeastern Mexico. The atlas covers all 3 known species, small fish around 1.5 in, each named for the drainage it comes from — the Maya gambusia (H. echeagarayi), Grijalva gambusia (H. milleri), and Coatzacoalcos gambusia (H. rachovii).
This is a genus of essentially scientific rather than aquarium interest: none of the three species is established in the hobby, and all three currently carry a Data Deficient conservation rating. That uniform Data Deficient status is itself worth noting — it typically signals narrow-range endemics that simply have not been surveyed enough to assign a firmer category, rather than fish known to be secure, and southeastern Mexico's river systems face real pressure from damming and water development.