Horses are extinct in the Fallout universe.
"Chris Avellone asked about whether horses survived the War in Fallout Bible 8: 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.'All roads is full of errors and is considered non-canon, it didn't even match what was said in fo:nv.
(the poor mule killed by the raiders in F1)
Excuse me, which one? And I stand corrected about All Roads and horses, didn't know it wasn't considered canon, or that it contradicted New Vegas so much. I'm a bit surprised horses appear in All Roads when Avellone was the writer.
There was a random encounter NPC that told you that. Tried to find it on the wikia but failed
On the graphic novel, I guess the guy drawing it just inserted it without consulting MCA (since the horses dont serve any significant role)