This is one of those difficult problems where I have a lot of respect for game designers. What's least unsatisfying?
1. Leaving it out altogether
2. A "brush horse" button that you click and that's the end of it
3. Making the player do it with the mouse
In American Truck Simulator you don't have to clean your truck, but you do have to get gas. Since a horse's cleanliness is more vital, maybe a cleaning mechanic (or at least a button/corresponding animation) should be in the game. Then you have to address the timing, which environmental/time factors, etc. would cause the horse to get dirty, if taking the horse into a lake will rinse it off (wait can our horses swim? Need to check with the dev team). I think I'd have a hard time designing a simulator that would actually be fun. So many trade-offs to consider.
I would say its not actually that difficult, its just constrained by whatever you're trying to accomplish. eg truck simulators dont have you cleaning, because its not part of the mental model of driving. But gas is a very necessary part of it.
Horse cleaning is important for maintenance, but a non-thought in travel. Thus a horse maintenance game would have brushing mechanics and it would be sensible, while it'd be a nuisance in a game emphasizing travel (even slow travel; feeding/housing your horse would be much more appropriate). But the key I think is to realize what the effect you're having on gameplay is pacing, and the mechanic of brushing or feeding or whatever is just an aesthetic/reason around it. So you'd only simulate so far as to render the pace of the game correct, and no more. So you might have feeding or cleaning, but probably not both in a travel game (it'd slow things down too much, and probably be part of the same operation anyways). A maintenance sim would prefer much slower pacing than even a slow travel game would
1. Leaving it out altogether
2. A "brush horse" button that you click and that's the end of it
3. Making the player do it with the mouse
In American Truck Simulator you don't have to clean your truck, but you do have to get gas. Since a horse's cleanliness is more vital, maybe a cleaning mechanic (or at least a button/corresponding animation) should be in the game. Then you have to address the timing, which environmental/time factors, etc. would cause the horse to get dirty, if taking the horse into a lake will rinse it off (wait can our horses swim? Need to check with the dev team). I think I'd have a hard time designing a simulator that would actually be fun. So many trade-offs to consider.