The site of blood running down a horse's leg is enough to make most of us feel a surge of panic. When a horse has cut himself the most important thing you can do is stay calm, and follow these triage steps.
Catch the horse and put him in a quiet, calm environment. You may want to give him some hay or grain to keep him quiet.
Then: inspect the wound. Use a clean (really clean) towel or sponge to gently wipe away the blood so you can see the depth, width, length and edges of a cut.
Note the following attributes:
-How long the wound is.
-How wide (you may even measure it with a ruler).
-Whether you can see any bone or tissue.
-If the wound has "flaps" that can be stitched.
-How fresh the wound is (is it still bleeding? Or has it clotted and dried?).
-Whether there are punctures associated with it.
Then, take your horse's vital signs and write all this information down so when you call your veterinarian, you can provide him with a full picture of the horse's wound and general health.
By providing these details, your veterinarian will know whether the horse needs stitches and antibiotics. If he doesn't need a vet visit, he might still need special bandaging, which your vet can explain over the phone.