From Fallout Bible #8:

Quote
1)Did horses (in any form) survive the Great War, within the area of California? Arguably, I imagine that if they did, horses would begin to replace cars as the main mode of distance transportation.

Answer: Nope. Some mules did (the poor mule killed by the raiders in F1), but not for long. Besides, I think that mule was a discrepancy.

That question is referring to the core region and not the entire United States. Horses may have well survived but do to the limitations of the gamebryo engine or other things, they have yet to be in the games

I read this book,i forget what,but it had oxen pull old ruined trucks that had the engine and stuff like that torn out,i would love to see a Brahmin dragging a truck around....what a sight!