Rattlesnake
Rattlesnakes, just like Copperheads and Cottonmouths, are part of the pit viper family. Thus they each have similarly large, triangular shaped heads. Aside from their colors and scales, these species have very similar head shapes.
While rattlesnakes have been associated with deadly defiance in American folklore — i.e. the “Don’t Tread on Me” flag — there’s something else to take away from that symbol: Rattlesnakes tend to warn those who come too close (by shaking their rattles).
The US is home to 16 species of rattlesnakes, all are venomous and all of them have the characteristic rattle. When it comes to geographic distribution, the rattlesnake’s habitat is virtually coast-to-coast, with a few exceptions (most notably Hawaii and Alaska). So no matter where you are going in the Lower 48, chances are you’re in rattlesnake country.
Thankfully, these very widespread snakes are also the easiest to identify, thanks to the rattles. The only outliers are juvenile rattlesnakes, which are (as it is in most species) a bit harder to identify, on account of their small, developing rattles and less viperish heads.