At some point in the past, I'd jumped on a horse that was tethered in a camp somewhere and rode it for a while before getting bored of it.
I didn't steal the horse - the name was in white when I jumped on - and now whenever I fast travel to an external location the horse appears next to me. It does, however, immediately start to wander off - presumably back towards the camp I found it at.
How do I get this horse to stop following me?
Preferably without killing it.