Bone Rattler

The sound comes first. A deep, rolling vibration like a rainstorm building on the horizon, except the sky is clear and the air is still. That sound is the bone rings around the tail of the largest venomous predator in Lone Star, and if you can hear it, you are already closer than you want to be.
  Bone Rattlers are the oversized descendants of the pre-Fall rattlesnake, scaled up by generations of mutation until the largest specimens reach thirty feet in length and five feet in diameter. They are born at roughly four feet, already dangerous, and they never stop growing. The bigger the prey in their territory, the bigger the snakes breed to match it. A Bone Rattler that feeds on Rumble Dogs stays manageable. One that has graduated to bison and horses becomes a threat to entire settlements.
  When not hunting, they are surprisingly docile. A Bone Rattler will spend hours coiled in the sun, basking, its tail rattle sounding off as a territorial warning to anything that might stumble too close. That rattle is the courtesy. When the snake is hungry, the courtesy stops. Larger specimens are drawn to ranches and settlements where horses and people congregate, and they have no hesitation about consuming whatever they can fit into their expanding jaws.
  The venom is potent enough to drop large mammals, delivered through a bite with reach that belies even the snake's enormous size. Ranchers and caravan drivers in Lone Star learn to read the ground for the wide, dragging furrow that marks a Bone Rattler's passage. The smart ones change route when they find one. The unlucky ones learn that thirty feet of venomous snake can close distance faster than a horse can turn.