The alien species has a complex developmental pattern that can lead to a variety of adult stages. Most aliens start life as a simple Multiworm Egg which in turn matures into a Multiworm. Multiworms eat like crazy and develop a bunch of internal... parasites? called Hyperworms. When a Hyperworm matures it proceeds to the Chrysalis stage.
And from there it's anybody's guess (maybe alien mommy knows) what's going to pop out of the Chrysalis... except that for sure it'll be a disgusting alien of one sort or another.
All of these related worm stages have their own attacks and means of locomotion. None of them are awfully dangerous by themselves. Unfortunately, they're rarely encountered alone.