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.
Steve, saddle up on the jingle
horse and
go jingle the other horses so they can be
fed before any of us can eat.