Edit: you talking about on the side of the tail..... hmmm How long has it been there? is it growing fast? keep a photo log of it and track it's growth.. might want to see the vet and maybe have it lansed?
That is strange, never saw that before. I don't think that could be a sperm plug, bumped out on the side of the tail like that???? Maybe if the bump was below the vent but I don't know about the side.
I would need a better photo to tell if it were a sperm plug.... if it's under the skin, not sure how it could be sperm. I think you might have a growth of some kind and should just keep an eye on it.