Horses have much better night vision than we do. They have a tapetum lucidum - a reflective layer behind the retina which is seen in most mammals other than primates.
Although they can't see in total darkness, horses can safely navigate in forest on a moonless night. They can manage quite happily if left to their own devices in an enclosed arena all night with no lights, without walking into the walls or each other and ending up injured.
The downside is that equine eyes do not adapt to sudden light changes as quickly as ours do. It's not uncommon for horses to spook or start when entering or leaving the barn (or a trailer) on a sunlit day. Also, horses can easily be dazzled by flash photography - it's not recommended to use flash close up with horses. (Fortunately, modern cameras can take some quite nice pictures without flash in fairly low light, especially if you have a tripod).
For writing purposes - a character who gets benighted in the woods might be able to drop the reins and trust the horse to get them home. (I've personally been on a horse in light where I would have wanted a flashlight if walking and just let the animal handle it).