How big of a spoiler is...

So my girlfriend read the books and we planned on going to see the movie when it comes out. I don't watch trailers or look at any marketing images/videos if I know i'm going to see a film, she also knows I dont watch trailers or like ANYTHING revealed.

Anyways this morning she told me >!the alien!< looks so good and I told her I didnt even know that was in the movie and that she spoiled it for me

Is it a major spoiler?