Because the helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils. Fragments of osteoderms from the Gila monster have been found in Late Pleistocene (10,000 to 8,000 years ago) deposits near Las Vegas, Nevada. The genus Heloderma has existed since the Miocene, when H. The evolutionary history of the Helodermatidae may be traced back to the Cretaceous period (145 to 166 million years ago), when Gobiderma pulchrum and Estesia mongolensis were present. There are three species in Mexico: Heloderma exasperatum, Heloderma horridum and Heloderma alvarezi, as well as another species in Guatemala: Heloderma charlesbogerti. The Gila monster has four close living relatives, all of which are beaded lizards.

They appear strong in their body structure with a stout snout, massive head, and 'little'-appearing eyes, which can be protected by a nictitating membrane. Body mass is typically in the range of 550 to 800 g (1.21 to 1.76 lb). The tail is about 20% of the body size, and the largest specimens may reach 51 to 56 cm (20 to 22 in) in total length. Its snout-to-vent length ranges from 26 to 36 cm (10 to 14 in). The Gila monster is the largest extant lizard species native to North America north of the Mexican border.

The first drawing of a Gila monster by Baird, S.

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