Is this horse stalled or out in a pasture? Are there other horses pastured with him if he's out? Is he lethargic or showing any signs of being "off"? Have you checked his temperature? Is the swelling smooth or does it have little "pock marks" in it? All of these are things you need to take note of.
It is possible if your horse is stalled that you are dealing with stocking up which resolves with work or turnout. If he is out in the pasture, I would look carefully at the leg for any small wounds, etc. You may be dealing with a blunt truama injury or an infection. If the swelling has continued for 3 days with no improvement, then I would have the vet out.
Registered Veterinary Technician