I've never felt that my horses were worried about riding in the dark. Its more my fears. I dislike getting slapped across the face by twigs and small branches. I can usually see the shadows of the larger branches coming at me. But the small ones sneek up on you.
I don't know what animals I am encountering in the darkness. As I said, I've frequently come across large black blobs and until they moo you don't know if it's a bear, moose or cow in front of you on the trail. I've heard cougars scream in the darkness. You know they are close, But can't see them. Where most cattle will herd away from a horse and rider. Buffalo don't and will actively charge a horse. Too many things I worry about.
The horses just plod along, and don't seem too worried about most of this. Branches sliding by their sides, don't bother them. They seem to be able to tell cattle from bears, probably they are smelling. Horses have more Rods in the eyes, So they have better night vision than we do, They give up the range of colors we see because we have more cones and fewer rods.
My rides are never on the road, So no worries about reflective gear or passing cars. Biggest concerns are safe footing for the horse and getting brushed off by a low branch.