The best answer I've found for being around horses is to simply slow down and stop well away from the animal. The rider will see you and act in the best way for that horse.
Do not yell or call out.
Do not ring a bell or blow a horn.
Do not do any brake lockup skids or anything similar.
You do not know the nature of the horse nor the skill of the rider. Just spare 30 seconds for a breather and let them go past nice and calmly.
There's a special case if you're approaching from behind to pass. The horse probably won't see you until you're near the back legs, and that's just asking for a startle. Absolutely do not use a bell.
In that case I'd calmly announce your presence to the rider without being too loud, and then ask if its okay to pass. If the rider requests you wait or anything, then you do as requested.
If the horses in your area are feral, they will run off long before you get near them. If there's a horse without a rider, then its possibly escaped from a paddock. If the horse is wearing a saddle then there may have been an accident already.
Pay attention to the horse's nostrils and how hard it is breathing. Horse's body language is fairly obvious; if it looks upset it probably is upset, so keep your distance.
There's no particular concern for lights or their brightness. Simply avoid shining lights at the horse's face. If it is dark enough to need lights to see, there's a much lower chance of meeting a horse+rider. A DRL should cause no problems.