Chronic Progressive Lymphedema
This is a very serious condition, and one not commonly found in Friesians specifically, but something that "very bad scratches" can be misdiagnosed as. If your Friesian has/develops large nodules around their pasterns, talk to your vet about getting a biopsy done to check for this disease. At this time, there is no cure, but research is ongoing. Recently it has been linked to being a genetic problem, so it is something that should be looked into and reported when a Friesian turns up with this condition.
Some helpful links regarding the above condition, with pictures and more information:
