The cottonmouth is a large and venomous aquatic snake. The cottonmouth earns its name from the white lining on the inside of its mouth. Their main defence mechanism is to open their mouth, exposing the white lining as a warning to predators.The colour of a cottonmouth is similar to that of a dark copperhead. The colour can be brown, black, or olive. Adult snakes range between 36 inches and 48 inches. These snakes are very aggressive, so keep your distance.