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. Or in a part of America not shown in any Fallout game (Such as areas like Indiana or North and South Dakota)

In All Roads, the graphic novel, every NCR soldier seen in Chance's flashback of the Bitter Springs Massacre are riding horses. Given the number of continuity errors between All Roads and Fallout: New Vegas it's likely this is merely another such occurence.