Highlander: Endgame, original trailer showed the villain, Jacob Kell, as some kind of sorcerer. Suffice it to say, they removed all of the “magic” in the released film. The trailer can still be seen with a quick YouTube search.
Amazing no one has mentioned Bridge to Terabithia (2007). The screenwriters disavowed the trailers, iirc. Looks like a cute kids fantasy movie, starts out leaning that way with a middle-school Manic Pixie Dream Girl, then turns hard into being completely real life and extremely tragic. 14-year old me was not prepared.
Godzilla (2014) The trailer showed it as a Godzilla movie starring Bryan Cranston. Instead Cranston only had a brief cameo at the start of the movie and the whole movie was about a couple MUTO creatures, not Godzilla. Godzilla basically only had a couple cameos in the movie. I think most people who didn’t like it just didn’t like being tricked by the trailer.
Man of the Year (2006) looked like it was going to be a lighthearted Dave-esque movie about a comedic everyman president finding common sense solutions through his hijinks. It pivoted pretty quickly.
Jurassic World Fallen Kingdom had a major part of the plot completely left out of the trailer, which I appreciated, but there were multiple suspenseful moments with the resolution in the trailer which was a bummer.