Longhorn Mudback Bison

For most of the year, the Longhorn Mudback Bison is the closest thing Lone Star has to scenery. Enormous, placid, and half-buried in riverside mud, these creatures spend their days grazing on tubers and vegetation along the rivers and coastlines, unbothered by the wasteland and largely unbothering in return. Their skulls alone are four feet wide, crowned with a pair of three-foot horns, and their hides are so thick and mud-caked that most predators do not bother trying.
  Then fall arrives. Mating season turns the passive giants into living siege engines.
  Bulls charge and ram heads to prove dominance, a contest that shakes the ground for hundreds of yards in every direction. The problem is that in their hormone-addled state, the bulls cannot distinguish between a rival bison and anything else that happens to be large and moving. Caravans, vehicles, iron horses, even small buildings have been mistaken for challengers and hit at full speed. Dieseljocks tell stories of entire supply trains scattered across the plains after a bull decided the lead wagon looked like it wanted a fight.
  Outside of mating season, they are valuable. Their size means a single kill can feed a settlement for weeks, and their hides provide material that no tanner in Lone Star would turn down. Ranchers who can manage herds without getting trampled during the fall months have a reliable resource. The challenge is surviving those few weeks when every bull in the herd is looking for something to headbutt, and your caravan is the most interesting shape on the horizon.