There are several similar species in the genus Dynastes, but only the Hercules beetle, Dynastes hercules is of a similar size (reaching 18 cm). D. hercules has:
Male Dynastes hercules a related species to the Neptune beetle.
The most closely related species is probably Dynastes satanas which is smaller than the Neptune beetle and is endemic to a restricted area of the Yungas in the Andes of Bolivia.
List of Dynastes species (after Móron, 2009):
Dynastes tityus - Eastern United States
Dynastes granti - Southwestern United States
Dynastes hyllus - Mexico and Guatemala
Dynastes maya - Mexico, Guatemala and Honduras
Dynastes moroni - Mexico
Dynastes hercules - Southern Mexico to Bolivia, eastern Brazil and the lesser Antilles
Dynastes neptunus - Andes of Venezuela, Colombia, Ecuador and Peru
Dynastes satanas - Yungas of Bolivia