You call a veterinarian who specializes in horses. If you try to treat it yourself it is likely to end badly. The vet will drain the abscess and administer an appropriate antibiotic.
Treats for horses:
carrots
apples
sugar lumps
salt lumps
turnips
If you feed your horse too much sugar lumps it will get tooth decay and gum disease unless you have a special toothbrush for...
A horses hoof is like a human's fingernail. It is made up of calcium. The horse has another "hoof" inside of the outer layer which is sensitive. If it gets infected, it may mean death to the horse....
If your horse has a wound that is infected, I'd definitely take it to a vet. That way they can give them antibiotics to stop the infection. Afterwards I'd suggest regular cleaning of the wound and...