Do you have mosquitoes? It looks very similar to what I dealt with a couple years ago when I had rabbits at my old house. I think it's Shope fibroma. That's how they start out, then they turn black before forming a scab and eventually they go away and the rabbit seems immune to it afterwards. It is transmitted through mosquitoes and fleas. If it isn't Shope fibroma, it may just be from a sting or bite, could be an abscess. I would first use neosporin and see if it starts clearing up and if not, use a drawing salve on it to get it to open up and drain.