Re: Horse riding in One Ring?
In what context do you need the skill? What's the drama? Are you thinking of chases, of remaining on horseback in times of stress like battle, or bonuses in combat while mounted, etc?
For the most part I just let people ride horses. If folks want to be good at it, they can say they are good at it. If hobbits or dwarves or whatever want to be poor at it, they can be poor.
As for coverage by the 18 skills, I think pretty well.