13. Eastern diamondback rattlesnake

Eastern diamondback rattlesnakes (Crotalus adamanteus) are the largest rattlesnakes in the U.S. and are found across southeastern states, including Florida, North Carolina, South Carolina, Alabama, Georgia, Mississippi and Louisiana. 

They are ambush predators and tend to lie quietly coiled away waiting for prey to approach. They can strike a victim up to two-thirds of their body length away, injecting a large quantity of venom with each bite. The species is not aggressive to humans, and bites tend to occur if a snake is intentionally harassed or accidentally stepped on, according to the Florida Museum. 

Their venom, which the snakes administer in about 75% to 80% of bites, kills red blood cells and causes severe tissue damage. If left untreated, the fatality rate for an eastern diamondback bite is between 10% and 20%.