Equidae

It is interesting to note that horses have never been shown or mentioned in a Fallout game, other than in the rantings of Dane in Fallout, and by Paulson and Sally in Mothership Zeta. This indicates that horses did not survive the Great War in North America, although there is a possibility that they exist in other parts of the globe.

Every NCR soldier seen in Chance's flashback of the Bitter Springs Massacre are riding horses in All Roads. Given the number of continuity errors between All Roads and Fallout: New Vegas it's likely this is merely another such occurence. During the flashback Chance has yet to be administered any drugs, eliminating the possibility it was a hallucination of his drugged mind.