He knew what he was looking for. The figures he was seeking would be thrillingly exotic, distinctively American, conveniently unclothed. Indians would do fine. Those in Brush's paintings have all the right accessories (beadwork on their moccasins, silver-studded belts, stone arrowheads, canoes), but they aren't convincing Indians. That's because they're stand-ins. Brush looked on them as "actors." They are stand-ins for the youths he meant to show us all along, the figures of the Renaissance, the gods of Greece and Rome.
The first warrior in a row of them in "Before the Battle" (1886) takes his pose from Michelangelo's "David." The second strikes a model's pose standard in the art school, though the pole that holds his arm up has been replaced by a spear.
Brush didn't ride a horse to Wyoming. He took the train, the new wide-funneled train. Life was changing clangingly in the 19th century--and rather than confront that change, lots of artists (not just Brush) chose to paint the pure-souled noble primitives of the pre-industrial past.
In France, Jean-François Millet painted pious peasants. In Britain, Sir Edwin Landseer painted kilted Highlanders. Paul Gauguin chose Tahitians. Such figures and Brush's Indians have a lot in common. They don't complain of poverty or gripe about injustice or dispute with their betters. Their presence in a picture reassures the viewer: Your soul is as pure as theirs.
Below: In "Before the Battle" by George de Forest Brush, the first warrior's pose evokes Michelangelo's "David."
P.S. A chief wearing nothing but a loincloth? I'm guessing that was rare.