Are you sure you really don't have my sister's horse in your herd? LOL
He had something like that too...we had no idea what it was, showed up one day, was hard but didn't bother him, and got bigger. Mom is a vet tech, so asked one of the vets at her office about it, and they said not to worry about it unless it got bigger, started oozing, or acted like it bothered him, and that if it didn't go away on its own we might biopsy it just to be safe. It eventually went away on its own, I don't remember how long it took it though.
Probably wouldn't hurt to call your vet and just ask their opinion, but they'll probably just tell you to keep an eye on it.