The implication is that the marines are only ever called to do pest control on colonies, that being taking care of not so dangerous indigenous life, those often being undocumented species, thus being attributed the name of Xenomorph, a name for any unknown alien species(Yes, calling our Aliens of Xenomorphs is infact wrong).
There is at least one sentient species they've made contact with, which are the arcturians mentioned in Aliens.