I wonder how many people debating this have actually sat on a horse.
Whether it was appropriate to this instance, I cannot tell.
A very good point. I have ridden, but no longer do. Some of my family are keen riders and I spend a fair amount of time around horses.
You're quite right about riding two-abreast being appropriate in some circumstances. These were not they. You're also quite right about the differences between riding a horse/pony and riding a bicycle.
However, I disagree about a rider having only to 'choose a destination and provide corrective input in case of problems'. There is a school of thought that a rider on the road should have both hands on the rein/reins unless signalling. Not everyone holds with that, but a fairly large majority do. It is certainly widely believed that a rider must be aware of traffic approaching from all directions.
I do agree about the rider having a plethora of sensory inputs should anything happen... Most riders are switched on and courteous. But in this case I was sitting/rolling at tickover in a fairly lumpy diesel for about 25-30 seconds behind the horses before either rider showed any awareness of my presence. As there are several stables in the area, we usually see horses out hacking in those lanes. The lanes are generally thought to be too narrow and sinuous for two-abreast riding. This is the view of horsey folk, not just this gnarled and jaded old car lover.
I agree with much of what you say and it is very clearly put. There was a lot of comment on this thread initially that I'd shown impatience in talking to the riders about the poor impression they were giving. I have to take that on board.
Nonetheless, I am sure that a rider on the highway must be completely in control of his or her horse at all times and fully aware of the presence of approaching traffic. These two were neither.
Thanks anyway for a thoughtful and thought-provoking response.