A picket line is a horizontal rope, along which horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s are tied at intervals. The rope can be on the ground, at chest height (above the knees, below the neck), or overhead. The overhead form usually is called a high line.

A variant of a high line, used to tie a single horse, is a horizontal pole attached high on the side of a horse trailer

Horse trailer

A horse trailer or horse van is used to transport horses...

. The attachment is designed so that the pole can be removed or folded against the trailer when not in use.