We recently had a bump that looks exactly like the last photo removed from our mare Cinnamon. The pathologist report said it was a benign (non cancerous) basal tumor. They are supposedly uncommon in horses, but can come from trauma to the skin, e.g. bug bites/stings particularly in horses that tend to have bite/sting reactions (not uncommon for our Cinnamon to get welts from bites).
We originally noticed it last June during bug season and it stayed about the size of a pencil. It didn't bother her, so we just kept an eye on it. This January, however, it doubled in size so we had the vet come out to remove it and get the pathology to insure it wasn't a cancer.
If it is a skin tumor, it won't go away on it's own, but the good news is that it is only on the skin and once removed, won't return.
Cinnamon's incision looks good...her hair is just starting to grow back where it was shaved, but it looks like there won't even be a scar. Total cost was $304 (farm call-$50, vet-$75, anesthesia/sedation/supplies-$97, pathologist-$82).
Definitely something you want to keep an eye on and discuss with your vet.