this is going to sound weird , but last year one of my show dogs had what looked like an abcess starting. We thought possible spider bite. Treated it with antibiotics, cleaned it out. This went on for two weeks, then all these little holes broke out. Still no pus or anything, but like the skin was eating itself up.
We sedated him, took a HUGE portion of neck skin out to make clean tissue to make sure we got it all. We started to suspect possible cancer. This was a 2 year old dog. . Anyway the vet soaked the wound in amoxicillan and since it was too big, we had to just let it heal. Sent in tissue samples.The report came back as an infection that is usually seen in cattle. Very weird to have it in a dog, and we have no cattle by us. I can't remember the name, but we then put him on Clavamox for four weeks and it eventually healed and closed up and no scar or anything., It was not contagious. The funny thing was as we were cutting tissue and cleaning it out, little green pieces of "something" were in the holes... It was just strange all the way around
I am mentioning it because his neck looked so much like what your horses cheek looks like.
You need a vet out to biopsy it as soon as possible. He will probably need to be on antibiotics for weeks to get rid of it, I imagine the vet might have to cut it out to get clean edges if its the same thing.