I'm happy to share the countryside with horses and their riders but in this day and age I wish they no longer need to use the roads especially if the rules ,such as lights, that apply to us cyclists either dont apply or are not enforced.
I too wish it was not a necessity to ride on the road, but as we all know there are far fewer bridleways than there are footpaths, so we ride where we can. It's ironic really when you consider that roads were made for horses and improved for cyclists that they now appear to be owned by cars and lorries.
As for lights, I'm not sure it a legal requirement, but it's recommended in the Highway Code. I have also had a few nasty scares / close calls with riders in dark clothing on dark horses at dusk. It has to be said though, that horses have excellent night vision, as anyone who has tried night shooting of rabbits (with the aid of a night sight) will have found out.
On the subject of poo! There is no requirement to pick it up, as just the same as cow poo it is just chewed up plants which carry no dangers to humans. Dog poo is different stuff entirely and around the village someone has put up some excellent A4 posters saying 'There is no such thing as the dog poo fairy'.
That said, allowing a horse to poo on a footway and not clearing it up is pretty rude and could get the rider in trouble.
Cars: A 14 hand pony (4'6" tall at the shoulder) can weigh over 500kgs, and horse can go closer to 950kg's. If that leans or sits on a car, then the car is an instant write off, if either kicked out with meaning, they could easily turn a wing or door panel inside out. It often troubles me how close people get to the back of us when we are walking, and I have been known to stop and look round at the driver to politely request they back off. I really don't think drivers realise how vulnerable their precious vehicle is.
"Try not to punch any clowns"