Cymothoa exigua is a salt water parasite that grasps the tongue of its host fish to begin feeding. The tongue dies eventually, and the parasite replaces the tongue staying put right there in the fish's mouth. A claustrophobic Cymothoan would have a pretty miserable existence it appears.
(The above pictures came from here.)
So, this unusual creature may not be an actual alien, but it was an inspiration for the creatures in the movie "Alien". Remember the mouth-within-a-mouth look of the aliens in that movie? (See picture below.)