The hooves are now painted in, although only three are showing so far. I spent a full morning on them.
I worked on the body to build up the paint layer a little and it's starting to look more finished. I also wiped some extra tone into the background to give me a better sense of where to take it. My original vision for the portrait was to have the background largely unpainted, but I've decided it needs something more substantial so I will probably use some thin mid-tone grey to help define the horse.