The four standard views
Start with the set that identification documents have always used: the whole horse from the left side, the whole horse from the right side, straight on from the front, and straight on from behind. Stand him on level ground in daylight, far enough back that his whole body fits the frame, with someone holding him rather than a loose horse drifting around the field. The two side views are not duplicates of each other. Markings are rarely symmetrical, a sock may appear on one hind leg only, and a scar or patch of white on the near side may be completely absent on the off side. A horse identified from one side only is half identified.
Close-ups of face and legs
Markings are what people actually describe when a horse is found or lost, so photograph them properly. Take the face straight on and at a slight angle so a star, stripe or snip shows its true shape and position, and repeat the shot for every leg with white on it, close enough that the top edge of each sock or stocking is clearly visible against the coat. Add close-ups of anything unusual: old scars, a splint, a patch where the coat grows differently, a blemish in an eye. These are the details a stranger can check in seconds, and they are exactly the details a general full-body photograph blurs away.
The individual markers nobody thinks to photograph
Two horses of the same colour and markings can still be told apart by features most owners never record. Hair whorls, the swirls where the coat changes direction, sit in consistent places on the neck, face and body, and their pattern is part of formal identity descriptions. The chestnuts, the horny growths on the inside of the legs, differ in shape from horse to horse in much the same way fingerprints do. Photograph the whorls on the face and neck, note which side the mane naturally falls, and take a clear shot of any freeze brand or hot brand. A microchip number belongs in the same file, recorded from a scan rather than from memory.
Coat, season and age all change the picture
An identification set has a shelf life. A winter coat hides markings under longer hair, a summer coat can bleach lighter in the sun, and a horse can look like a different animal clipped than he does unclipped. Young horses change fastest of all, filling out and sometimes changing shade as they mature, and greys lighten year by year until last year's photos describe a visibly darker horse. Retake the set when the coat changes with the seasons, after any injury that leaves a mark, and at least yearly for a growing youngster. Label every set with when it was taken, and keep the newest set as the live one.
Keep the set where it can be used
Identification photos are only useful if they can be reached at the moment someone needs them, which is rarely when you are standing next to a filing cabinet. Keep the current set, the microchip number and the brand record together on his KinsLe Pet Pass, and the horse's verified identity can be shown to a vet, a transporter, a yard manager or a finder straight from your phone, with the photographs to prove the horse in front of them is yours.