Just like the title says: in what episode of the anime does Natsu become a demon (E.N.D)? I was wondering if that even happened in the anime or only in the manga?
The current Anime Episode is shortly after the Tartaros Arc.
Right now it is unknown to Natsu that he has something to do with any Demons inside of him.
The last episode of the Tartaros arc is where we learn that Natsu is E.N.D. Zeref is holding the Book of E.N.D. at the end of Episode 265 when he pronounces Natsu's full name: Etherious Natsu Dragneel.
However, after this, the anime went to Fairy Tail Zero material, which relates the backstory for Mavis, Zeref, and others.
Put simple, Natsu has not become a demon yet in any form of media. It has been revealed that he is END in both the manga and anime, but Even in the manga he has not transformed. It may never happen.
In the manga Natsu turns into his demon form in chapter 500 and it is revealed that he is E.N.D after his older brother(Zeref) reveals his full name: Etherious Natsu Dragneel at the end of the Tartaros arc. Although his demon form has not been revealed in the anime and Natsu has no clue about what he truly is but it is highly anticipated that it will show up... eventually.
I'm pretty sure that the show wouldn't end without Natsu finding out that he is not only E.N.D, but that he is Zeref's Brother who passed away 400 years ago when they were only little kids. One person that should be included (if the show digs deeper into Natsu's background,) is Larcade Dragneel. I'm not really sure what his connection is with Natsu, although it would be a nice addition to the show and it would keep things interesting.