That's just one of the gargoyles grotesques on France's Chapelle de Bethléem, which is a legitimate medieval chapel near Nantes, France. When the chapel was being restored in the mid-90s, they discovered that the old architectural drawing from the 19th century didn't specify what the sculptures were, so they kind of had free rein to update it as they saw fit. In addition to the Alien (which represents the Biblical Leviathan), there's also a Gremlin, Gizmo, and Mazinger Z as "the knight of modern times." You can see more photos at Geek Tyrant.
If you were in charge of geek-ifying a church, what characters would you put on it, and what would they represent?