Despite the common name "least killifish", it belongs to the family Poeciliidae and not to one of the killifish families.

Heterandria formosa (Dwarf livebearer, least killifish)

Heterandria formosa (known as the least killifish, mosqu or midget livebearer) is a species of livebearing fish within the family Poeciliidae.

Heterandria formosa (known as the least killifish, mosqu or midget livebearer) is a species of livebearing fish within the family Poeciliidae.

Heterandria formosa (known as the least killifish, mosqu or midget livebearer) is a species of livebearing fish within the family Poeciliidae.

Most species occur in Guatemala and its surroundings, particularly Mexico, but the midget livebearer (H. formosa) comes from the southeastern United States.