How to Give First Aid to a Horse
If you need to give first aid to a wounded horse, start by cleaning the wound with mild soap. Then, flush the wound with water and cover it with gauze pads. If your horse has a swollen hoof, check the underside for a puncture wound and remove any foreign objects that you see. Next, fill a bucket with Epsom salt and lukewarm water and have your horse stand in it for 10-30 minutes to reduce swelling. If your horse has tack sores, trim the hair around the sores, rinse the area with clean water, and smear a thick layer of antibiotic ointment or petroleum jelly over it to prevent infection.
Check your horse's temperature. Grab a 6-inch veterinary thermometer, and shake it out till the temperature reads somewhere around 95 degrees. Add a bit of petroleum jelly as lubricant to the end of the thermometer, and clip a clothespin to the other end for better grip. Stand just to the side at your horse's rear, and insert the thermometer into their rectum. Allow it to set for 2-3 minutes, and then remove it and check the temperature. The average temperature of a healthy adult horse is 99-101 degrees. For a foal, this is slightly higher at 99.5-102 degrees. A temperature change in 1-3 degrees is no problem, but if your horse's temperature has risen 4 degrees or more there may be an issue. Remember to consider things like hot weather or a heavy workout before jumping to sickness as the cause of your horse's rise in temperature. Take your horse's heart rate. For the easiest way to check the pulse of a horse, use a stethoscope. Hold the end of the stethoscope to the horse's chest, just behind the left elbow. A single 'lub-dub' is a single pulse; time yourself for one minutes and count the number of pulses you hear. For a healthy horse, the resting heart rate should be somewhere between 35-42 beats per minute. Anything much higher or lower than this may indicate a health issue. Check their respiratory rate. Make sure your horse's breathing isn't out of sync by checking their respiratory rate. Stare at their rib cage, and watch for it to rise and fall. Time yourself as you count the number of times you see their rib cage expand over the course of a minute. A healthy horse will have a respiratory rate of 8-20 breaths per minute. If your horse has recently exercised, has been in hot or humid weather, is pregnant, or is very old, they may have a higher respiratory rate without being ill. Test your horse's hydration. When it is especially hot, your horse has been working very hard, or you are concerned about possible illness, check your horse's hydration levels. Pinch a piece of skin from their chest or shoulder between your forefingers, and then let go. If your horse is hydrated and has plenty of fluids, the skin should return to its normal state in a second or less. If your horse it mildly dehydrated, it will take 2-4 seconds to return to normal. Severe hydration is indicated by peaked skin lasting 5-10 seconds. Check their gum color and capillary refill rate. If you're worried about your horse experiencing a toxic reaction, shock, or severe hydration, test their gums. Lift their lips and apply pressure to a spot on their gums with your finger, waiting until it turns white. Then, release the pressure and count how long it takes to return to its normal pinkish hue. A healthy horse will have color return in 1-2 seconds. If it takes 3-4 seconds (or longer), your horse may be experiencing health problems and you should consider calling a vet. If the gums are not pink but red, your horse may have consumed a toxic substance. If the gums are whitish in color all around, your horse may have had blood loss or may be in shock. If the gums of your horse are blue or turning blue, contact your vet immediately as it is a sign that they are close to death. Treat a laceration. Rinse the wound thoroughly using light pressure from a garden hose, or ideally, flush with clean water or betadine/chlorhexadine solution(diluted)using a 60cc syringe with an 18 gauge, 1 & 1/2" needle attached to the syringe. Following flushing, lightly scrub with betadine or chlorhexadine surgical scrub if handy. Apply several gauze pads over the wound, and cover them with one or more large quilted pads. If you don't have chlorhexadine surgical scrub, Ivory dish soap or any other mild soap will suffice. Bandaging wounds on a lower limb are easy, but anything higher (stifle, shoulder, hock or upper body) are difficult places to keep a bandage in place. If the wound is on the lower limb, apply a clean telfa pad, then wrap a quilted pad and an elastic bandage, and make sure it is secure, but not so tight as to interfere with circulation. Treat any swelling from a kick or fall. Have the horse stand still with the swollen leg in a bucket filled with Epsom salts and lukewarm water, if the swelling is on the lower limb. If the horse is fidgety and will not stand still, you can dip a quilted stable bandage in the Epsom salt water and bandage the leg to reduce the swelling. After the horse has been standing with its leg or hoof in the bucket for about ten to thirty minutes, apply a poultice, either homemade or store bought to the swollen area Treat an eye injury. If you notice pus or discharge in your horse's eye, your horse has a swollen eye, or you see a small object stuck in their eye, treat it at home to relieve your horse of their discomfort. Remove any objects or items that you see that are stuck in the horse's eye. Then, use an equine eye rinse to flush the eye and remove any discharge. You can follow up with a soft, damp rag to wipe away any remaining discharge or solution. Treat a puncture wound. Most typically found on the hooves of the horse, punctures can be dangerous because they appear to be minor while they are in fact very deep. Refer to your vet for the treatment of puncture wounds, but in the mean time remove the object (and save it) and clean the wound with clean water. Then, have your horse stick their foot in a bucket of warm water that has epsom salts dissolved into it. If your horse won't stand with its foot in the bucket, you can soak a quilted pad and wrap their hoof with it. This may be more difficult though. Treat a tack sore. Ill-fitting tack can cause minor sores and cuts to appear after a long or strenuous ride. As long as the sore isn't too deep and isn't infected, treatment at home is appropriate. Trim the hair around the sore, and then clear the area with clean water. Clean it out with betadine and smear a thick paste of antibiotic ointment or vaseline over the wound to keep it from getting infected. If you're out on the trail when you notice the sore, cut a piece of 2” foam in a donut shape around the sore, and tape it to the bottom of the tack. This will prevent the tack from rubbing the wound and making it worse as you continue to ride. If you notice tack sores, have a professional help you determine if your tack truly fits and ways that you can improve its fit. 
Check your horse's temperature. Take your horse's heart rate. Check their respiratory rate. Test your horse's hydration. Check their gum color and capillary refill rate. Treat a laceration. Treat any swelling from a kick or fall. Treat an eye injury. Treat a puncture wound. Treat a tack sore. 