Warmth and tightness are from the swelling, not necessarily an infection which is what penicillin is for (I am sure you know that -- I am terrible about stating the obvious sometimes). With a deep abcess that blew out, he was not lame? If you suspect snake-bite I would get the vet out. One of the horses at my barn got bit just under the fetlock and she has been recovering for more than a year. Something about where the bite was caused alot of tendon problems.

You are right to cold water hose the swollen leg, IMHO I would just call a vet before giving antibiotics.