This is what happened a couple of weeks ago, quite suddenly, out on a ride. It's the inside of Brena's right hind foot. She remained fully sound and her hood capsule maintained full integrity.
Last week Brena stepped on the edge of the trailer ramp, removing more hoof. Again she remained sound, and was seen cantering around her field. By this point she was not being ridden, and I was getting quite concerned.
Today a good trim made the damage look a lot less fearsome. It seems like the damage mostly affected a noticeable flare in the hoof wall, which hadn't been rasped away at recent trims for some reason. It doesn't extend as far as the sensitive laminae, which is very fortunate. The residual area of damage needs to be cleaned daily with surgical spirit until grown out. Brena is still fully sound and moves forward freely.
On her hinds, Brena does move by putting weight on the inside and rolling to the outside, rather than heel to toe. It's a factor of the slightly off conformation that she always seems to have had.