This is a very valid option. Let's expand it a bit further. Most mobs especially later would make the horse unmanageable and it basically throws you if you don't quickly dismount. If you dismount, you can't just tend to the horse and fight mobs. So the horse will run away. If thrown, you take some small damage but mainly get stunned for about 2-3 seconds. This gives you a real negative to just going riding wherever you please. Horses cost good money and are an expensive loss. Losing your life because you ran into that wondering Hill Giant and he flattened both of you can be very costly.
The benefits of horses are on roads. Have one major highway linking the cities on the continent. Have a few wandering guards patrol portions of the road until the end of that cities' territories. For large portions of the road, you can safely travel to meet with friends who are stationed far away. It gives you another option to portals and gates that can be fairly quick. You could ride the horse halfway down the road and get to a certain spot you want to a bit quicker than other options possibly.Horses can move you through easy content in certain areas in the game. For instance, a high level who has good standing with gnolls, can move through their territories quickly and easily later in the game. The way to keep horses out of the early game is riding skill reqs. Expense also limits their use. Lol, it's just an idea so don't be too harsh people.