The placement of the bare patch is a bit unusual. Sore hocks/calluses are a result of friction, so unless your bun has some sort of disability that makes her put pressure on the side of her foot when she walks/hops, this seems like sth else. Could she have snagged it on sth? Or done it to herself? Perhaps in an attempt at removing a piece of stuck hay? Some bits of hay can be very sharp, like splinters, I’ve noticed myself. They can easily penetrate the skin and cause ridiculously sharp pain. How does the skin look? (Wet, dry, color, any scaling or swelling etc).

Hmm, it does look like a sore hock starting to form to me. A callus is usually right on the end of the heel. The other option could be a hay splinter, as Bam suggested. My bridge bunny Bertha got a sore on their hock that ended up being a tiny abscess from a splinter.

At this point it doesn’t look too bad though. I would focus by making the flooring softer, especially in the areas she spends the most time. Carpet can be pretty abrasive, unfortunately. I just went through this with one of my buns, and I ended up getting a memory foam mattress topper to put under my bun’s pens, and then topped that with washable pee pads and a very soft blanket (those polyester ones that feel like super soft fur, if that makes sense?). You can also get fleece fabric from the fabric store, and try add a variety of surfaces and textures. Giving the bun uneven surfaces (such as crumpled towels) can give opportunities to take the pressure off of hot spots. I’ve seen some people rave about those chenille bath mats for sore hocks. You don’t have to change everything all at once, but start adding softer textures where you can in the areas where she spends the most time.

That looks to be more consistent with a bunny being kept in a small cage and the side of their foot rubbing on the side or bars. How long have you had her? What were her living conditions like before she was at the shelter.