Zeiram

Zeiram is one of the first unstoppable killing machines in history. Their genetic origin is unknown. All that Humans know is he's an unkillable space creature that can't be stopped by conventional means, although its core seems to be its weak point. This alien has adept fighting skills and incredible strength and adaptive intelligence. While only one is known, one can assume that, if it is a species and not some sort of creation, that there would be several to exist, even if only one Zeiram currently remains.