Thought I'd give an update! Thank you for everyone who gave suggestions. It left me paranoid, but I think better able to asses the vet's diagnosis.
The first vet did still think it was rainrot even after coming out, looking, and cleaning it. I didn't agree with her at all. Got some antibiotics for Cally at least & some fungal/anti-bacterial shampoo. Now I did agree with her that if it happened to turn out to be something bacterial we didn't want to cut into it & therefore risk spreading the bacteria further.
I also had the barn's farrier check it out. Also said he didn't think it was rainrot. Which I just now realize I failed to mention she did have two small bumps on her back in the exact spot this is at. We thought that maybe the bites had itched pretty bad, she kept rolling to scratch them, they either burst (or ya know how kids keep scratching mosquito bites & they turn into little craters?) or got irritated - then infected from flies getting at the thing. He said if there were bugs in there to try a power pack wormer.
Then had another vet look at her. She wasn't exactly sure what it was, but said if it was from bugs it would be slimy. Aside from those pictures with the puss - it never was slimy. I hadn't been coming out to the barn due to work & had another girl riding her so didn't see this when it first started, but cleaned it EVERY day once I got back to the barn regularly. Never once thought it was slimy.
BUT the good new is that it's getting better. The antibiotics are helping. I'm not so sure about the shampoo, but I still use it since it should help cleaning it off. The edges are starting to come in and I've seen no puss.