My lovely 'bomb-proof' horse (see left) got hit by a car who didn't see us in broad daylight (August), please tell the girl from me that it isn't much fun watching the horse you've loved and cared for for all those years (17 in my case) bleed to death by the side of the road, with traffic and sirens everywhere, quite traumatic for all concerned actually (and the driver did get done for dangerous driving incidentally).
Anyway, as for riding in the dark I'm sure it is law that any road user (bike or horse) must have a forward and rear facing light in the dark. Personally, I think it should be law for horses and cyclists to use a hi-vis tabard on the road whatever time of day, as I've learnt the hard way that there are plenty of pillocs out there who drive round without paying attention to what they are doing. A hi-vis tabard of the type that workmen wear costs less than £5 from any work wear or car supplies shop. I have a friend that never wears anythig hi-vis so I keep donating ones as a hint!!