I do not know how long ago I rented this horse to test something or who I even rented it from. But it is stuck in my stables and there is no way to return the horse to its owner. Please fix it so the renter can send the horse back so it is not taking up slots. Please.

Totally not recommending this, but what happens if you create a throw-away character, take that mount out, then delete the character without putting the mount back? Does it get returned to a stable? Can you even take that out in the first place?  Transport it to nowhere (like Muiquun)?