Lonesome Dove, Volume 2: Comanche Moon by Larry McMurtry

BLUE DUCK let Ermoke and Monkey John ride his spare horses because of the two Comanches who watched them for a day. Ermoke was the first to see them; it was shortly before his horse gave out. He pulled his rifle and pointed to the west, but Blue Duck, at first, could see nothing that he could clearly identify. Monkey John, so shortsighted that he would sometimes climb on someone else's horse thinking it was his own, could see nothing, but he pulled his rifle just in case.

"What you see is a yucca, or two yuccas," Blue Duck told Ermoke. He was anxious to press on and catch up with Buffalo Hump, whose track was the track of a weak old man-a man who would die within a day or two. Blue Duck did not want his father to die before they found him. He was prepared to ignore everything else in order to catch his father before he died.