It was an experiment in genetic engineering, to create a way to overcome the limitations of our bodies as put in us by evolution. An attempt to explore the limits of what DNA can really do and the experiment was a success - that's where things started to go wrong.
His name was Tommy Cooper, a convicted rapist who cut a deal with the Omnidyne Corporation. They do everything in their power to get him out of prison and into their labs, he lets them use him as a lab rat in a quick experiment, and then he goes free. Of course they haven't told him that they don't expect Tommy to even survive it - none of the other dozens of test subjects have. As long as they learn something every time they're really fine with it.
This time the experiment is their first real success, Tommy survives the process - then he begins to mutate out of control. Turned into something that really can't be called human anymore it escapes containment and is loose in the facility, driven really by only his baser instincts. The urge to be free, the urge to eat, and the urge to mate. They'll eventually slay the beast that was once Tommy Cooper - but not before Tommy successfully plants his seed in some unfortunate female victims. . .Requirements:
Rather straightforward really, we can discuss the long term aspects of this one. Like the creature manages to successfully seed multiple female employees of Omnidyne before its killed, or it escapes out into the world. We can discuss attributes of the creature as well though having it be vaguely humanoid would be preferred.