A horse trailer butt bar is a padded metal bar (or sometimes a padded metal chain) at the front or back of the stalls inside a horse trailer. Horse trailer butt bars are designed to keep your horse in the correct stall area, but if your horse becomes stressed and decided to rear up and kick out, he could get trapped on top of or underneath the butt bar. For that reason, you should only use trailers with removable safety pins, a horse trailer divider pin, or quick release features on the butt bars. The best option is a trailer that uses only horse trailer dividers without butt bars.