"If you look at the variation in species on this very Earth the Alien is totally a thing that could exist."
You mean, like the hardiness of tardigrades, the pharyngeal jaws of moray eels and the ability of wasps to use host spiders to breed inside. Sure. But each one of these animals only individually displays traits of the Alien.
Oh, you could combine all of them into a single specimen and you would get the Alien.
...but that's not exactly natural, is it?
-Windebieste.