To answer your question, @yickenscarf23, I buy the more expensive, but harder plastic mats. Plus, my rabbits have sticks and pine-cones to chew on, so they usually are pretty good about not chewing litter boxes, food bowls, mats, etc. They also are let out into ground pens on warm, dry days.
I think it is probably sore hocks, which appears to be a raw, bald patch of skin on the rabbit's hock. this is caused when a heavier rabbit lives in a hutch or cage with a wire bottom. All of my rabbits have cage mats to prevent this from happening. I would guess it's not frostbite because...