Hair Loss Bald spot on butt

[Doorman]

Recently I was looking at my guinea pigs play and noticed this huge hole on one of my guinea pig!!
He doesnt seem to be hurting or anything and my other piggie doesnt seem to have anything similar.
What is this condition and how do I treat it?

 

[bpatters]

I can't tell by the picture whether or not it's a swollen lump. If it is, he needs to see a vet ASAP. Is there an injury there, like a bite or scratch?

If it's not at all swollen and there's no injury, it's possible that's it's ringworm. I tend to doubt that, though, because while ringworm causes that round hair loss, it doesn't leave the skin shiny and smooth underneath, as that appears to be.

If it were my pig, I'd see a vet.