I bought the black horse from the black waterside stable outside cheydinhal and then ran over to the fort to get shadowmere. It's a thing about asthetics, you know, realism, like having a couple of horses at one stable. If you leave a horse at the stable where you bought it and buy another horse at another stable, the unused horse stays at the stable where you bought it. For example If I buy a horse at black waterside stable and leave it there, then I not fast travel but actually walk/run all the way to another stable and buy a new horse, the new horse goes everywhere while the first horse stays at black waterside stable. My problem is my blackwater side stable horse did not stay at battlehorn castle when I ran all the way to the fort where shadowmere is and then I fast travelled back to battlehorn. And I don't see the horse at blackwater side stable where I bought him originally. I'm not experienced with the cs so I don't want to mess anything up.