To me, the scariest movie or book alien is the cute yet devastating tribble. You'd be starved as they reproduce on your food supply, and they'll purr at you while doing it. It is so plausible. Take a creature out of it's natural environment and all heck breaks forth.



In real life Australia had a 'cute & fuzzy' rabbit explosion that nearly destroyed the country, as the animals had no enemies, literally eating other critters out of house 'n home.