There really isn't any point of waiting to see what random people on the internet think it could be. I'm not a vet, and even if I was, I can't diagnose something from pictures. It could be a wound caused by running into a stick in the yard. It could be a ruptured cyst. It could be a cuterebra. If could be an abscess from a spider bite. It could also be something else completely. The longer it goes untreated, though, the longer it's going to take to heal.