This should be something, I don’t get why we can’t.
In which RPG can we talk from horse? 
If you’re on same level, think you can
Well, I hate to even mention it, but several games in the Elder Scrolls series at least allow NPCs to accost and speak to you while you are mounted.
I was waiting for the answer: Witcher3…but wait… 
This would be cool feature but it is even issue when someone is sitting and NPC stands up and his head is out of screen. Similar can happen.