WTH is this?

[JLC]

Wouldn't a vet have recognized a burn? Or scale rot? And if he rules out those two issues, why would he not test for a fungal infection or at least try a fungal treatment to see if it helped?

If your vet, who holds the snake in his hands and looks at it with his own eyes, can't tell what it is, how in the world will any of our guesses help? It's been suggested that you have the vet TEST for a fungal infection. Yet you keep asking for a new diagnosis? As if maybe someone guessing something simple to treat will suddenly make this a simple matter?

You don't need a heat mat with a corn snake. You just need to keep the ambient temps around 75-80...which can be done quite easily with an overhead lamp if the room its in is cooler than that. I'd ditch the mat entirely and get a simple bulb, or a space heater to warm that area of the room. (Which would be helpful for your other snakes as well, if they're in the same general area).

Without a heat mat, you don't have to worry about him burrowing under his substrate, whatever it is. Astroturf is even worse for holding onto and cultivating excess bacteria than aspen is. We've used aspen for seven years with our gopher snake (who is also a burrower), with the spot-cleaning method...and have never had a single health issue. I do agree that keeping a snake with skin issues on paper is BEST...but I don't believe the aspen was the culprit unless it had gotten particularly wet and nasty for long periods of time.

I do hope you figure it out. It looks like a burn to me, but that is not an accurate guess at all because all we have to go on is a few pictures and none of us are vets. Keep up the antibiotics, but I would HIGHLY recommend that you follow Derek's advice and have the vet do some culture tests. I would NOT suggest treating the matter with human anti-fungal powders or creams. You don't know how those might affect the snake.

I also recommend, again, that you ditch the heat mat entirely and simply warm the entire cage to 75-80 degrees.

I'm sorry I can't give some definitive answer or any simple home-remedy treatments. Maybe just removing the potential burn source and allowing the area to heal on its own will be enough. I hope so.