[ml1201]

I'm going to go with an experiment that had escaped that had a mutated strain of the virus that allows it to camouflage itself, making it seem invisible. It had been just laying on the side of the road invisible when it heard the two of them approaching, so it just waited for them to get closer. Since it saw a chance for an easy meal (Akio being hurt and tired) it decided to take a shot at killing him. The growl was when it was still invisible, it didn't become visible until after it got impaled on Tabitha's shield, having made one just in time after hearing the growl. Does that sound good?