See quotations.
1869. [In the Far West and on the Plains,] everything is an outfit, from a [railway] train on the plaines to a pocket-knife. It [the word] is applied almost indiscriminately,to a wife, a horse, a dog, a cat, or a row of pins.A. K. McClure, Rocky Mountains, p. 211 (Bartlett).
1870. In company with a Mormon outfit of sixteen men, ten wagons, and sixty mules, I had made the wearisome journey from North Platte across three hundred miles of the American Desert at the dryest season of the year.J. H. Beadle, Life in Utah, p. 217 (Phila., &c.).
1887. The American herder speaks of his companions collectively as the ranch or the outfit.Louis Swinburne, The Bucolic Dialect of the Plains, Scribners Mag., ii. 509/2 (Oct.). (N.E.D.)