A horse kept at a campsite overnight, to be ridden in the
morning to round up (to jingle) all the other horses who are out grazing.
At a cowboy campsite, most of the horses are allowed to
go graze overnight with some degree of freedom, with a little bell attached so they can still be found in the
dark by the sound. One horse is kept near the camp so it can be ridden to round up the others in the
morning.
Traditionally, the horses were supposed to be fed before anyone else could eat
breakfast, so someone would be tasked to go "jingle" the horses in the early hours of the morning, since the little bells would jingle whenever they moved.
Source: Equus Magazine, "The Language of Horses" Nov 2014
The phrase also appears in the original lyrics for the Christmas song, Jingle Bell Rock.