Just as the title says, a way to disable all the horses you see about in towns. Kinda blows the vibe of a town when the whole area is covered in horses. 

They should have hitching posts around. When you put your horse on it, only you can see it. While horse is on it, it should regen or get some stacking buff for staying hitched for up to 10min or something, to encourage its use.

This feels more elegant as a solution than "hide shit".

You can stable your horse and walk into town.  Or when you go to see some NPC, RP it a little and don't ride right on top of a king or queen or even storage keeper.  Try to place mounts where they more or less make sense from an immersion standpoint.

But that's on the players as a community to do or not.  No additional features are needed.  In fact, hiding all the mounts would look really odd.  There's also nothing to prevent people from doing things like, say, putting elephants on roofs, and claiming that it's perfectly normal and not at all disruptive. Except for maybe common courtesy, but that's just so old fashioned.

I would say hiding mounts should work same as lot of us wishes with ships parked at wharf. If half of them belong to people afk fishing then after certain time of inactivity, game should forcefully remote collect it to nearest stable. Be it horse or ship. Otherwise yes, its on players and players only that they take their horse everywhere but then again lot of people proven in real life, that if they can drive all the way inside the building, they will.

I see no better and simpler solution for now than just hiding horses belonging to afk players. Give them 30 mins of inactivity warning  (which would be not mounting the horse+not moving on it) and their mount gets remote collected. I don't think anyone needs more than 30 mins to sort gear/loot/have a short break from grind, whatever. People lifeskilling and cooking in their houses don't need their horse outside either so it should be always inactivity timed of not using your mount and ofc only in major cities.