I just had the craziest glitch! I was at Beecher's Hope and replayed the mission "Horsemen, Apocalypses" to kill some O'Driscolls...when the mission was over and I was back at Beecher's Hope, I couldn't believe my eyes! There was Old Boy (NPC John's horse in 1899) randomly chilling between the barn and the house! I could study him, mount him and hitch him but not bond or saddle him (like a Mule/Donkey).
But how's that possible? I mean Old Boy was at camp when the O'Driscolls attacked, so this might have caused the glitch...but idk why I'm able to mount him xD