It is one of the island nation's few large land animals, and it is the largest endemic terrestrial lizard in the country, and one of the biggest rock iguanas within the Antilles.

[4] Its specific name, stejnegeri honors Leonhard Hess Stejneger, who, when writing his Herpetology of Porto Rico in 1902, suspected the rhinoceros iguanas of Mona Island might be slightly different in scale count than those in Haiti, although he was unsure.

Males possess bony, prominent tubercles on their snouts resembling horns, adipose pads in the form of a helmet on the occipital region of their heads, and large dewlaps.

This subspecies, like other species of Cyclura, is sexually dimorphic; males are larger than females, and have more prominent dorsal crests, "horns" and femoral pores on their thighs, which are used to release pheromones.

Thomas Wiewandt, who spent an extended period on Mona Island studying C. stejnegeri, suggested that the horns, along with lateral spines and prominent parietal bulges, function as protective armor against sharp rocks or as defensive tools to facilitate the escape of males from the grasp of one another.

Although Mona ground iguanas use the whole island as their habitat, only 1% of the territory, located on the southwest coast, is suitable for nesting because it contains loose sand, and receives direct sunlight.

[13] A study in 2000 by Dr Allison Alberts of the San Diego Zoo revealed that seeds passing through the digestive tracts of Cycluras germinate more rapidly than those that do not.

[4][15] The Mona ground iguana is also an important means of distributing these seeds to new areas (particularly since females migrate to nesting sites) and, as the largest native herbivores of their ecosystems, they are essential for maintaining the balance between climate and vegetation.

[4][15] Their diets are very rarely supplemented with insect larvae, crabs, slugs, dead birds, and fungi; individual animals do appear to be opportunistic carnivores.

[15] This health screening has been used to provide baselines of the normal physiologic values of the species, identifying potential future problems due to parasites, diseases, etc.