Some petroglyphs that illustrate horses seem to also indicate a mark on the body of the animal. In some instances that probably indicates the paint on the horse that warriors of Great Plains tribes are known to have applied when they are faced with combat and have adequate time to prepare.
This example of a ledger book drawing shows Cheyenne warrior Brave Wolf fighting against a Crow warrior on horseback. The leading cavalryman has obvious brands on his horse, and Brave Wolf's horse shows a brand very much like that in Keyser's compilation of brands shown above.
Plains biographic rock art panel,48HO9, Keyser, 2012, p. 14, fig. 5
Keyser was an early proponent of comparative interpretation in rock art. He pointed out back in the 1980s that elements of Plains Biographic Style rock art matched elements found on painted robes and in the illustrated Ledger Books. His insights and amazing body of published work has made an immense contribution to our study of rock art.
McCleary, Timothy P.
2008 Ghosts on the Land: Apsaalooke (Crow Indian) Interpretations
of Rock Art, PhD Dissertation, University of Illinois,