Sounds like a warble to me. If you "google" warble, you find two definitions. One is for a worm, a fly larva, that is under the skin. This is usually found in cattle or deer. I've never heard of warbles in horses, but that doesn't mean a thing! According to the research I did in about five minutes, injectible ivermectin will kill the warble larvae. But whether oral will or not I can only surmise.
Another definition for a warble is a hard tumor on a horse's back, caused by friction, but this doesn't apply here.
I've seen warbles in deer and in squirrels. The worm will eventually eat its way out through the skin, leaving a wound that can become infected. We don't eat meat from deer or squirrels with warbles.