Although your horses swelling is much larger than my horse's is, they sound similar because my horse is in no pain at all whenever I apply a lot of pressure. We figured the wound was from a horse fly bite. It's been there since the begging of summer and has showed no signs of retrieving. If it is indeed a horse fly bite something like Penicillin might be necessary if it's an infection. Be very cautious when giving penicillin shots one wrong hit and it could kill your horse.
"Unbridled or tethered and tied, the safety of the fence or the danger of the ride, I'll always be unsatisfied."