It is hard to say what it is from a cyber space description but it could be
tendon (if posterior) or a splint (if more medial) or just a blow. Trauma
from a blow to the leg usually results in periosteal (connective tissue
covering of bone) bruising which can be very painful if touched but not
load bearing and will eventually heal with a bony swelling similar to a
splint. If the horse is having problems, I would definitely recommend
calling your vet. You can also use some arnica, both topical liniment and
oral homeopathic. But get a good diagnosis first.
Once you know, try magnets too, see my article in last issue of Trail Blazer.