Monsters, being naturally interesting, often appear in movies... well, duh.
However, there are a rare few monsters who either actually started as movies that are now somewhat forgotten, or had a major change to the ideas about them made by a popular movie -- with this second factor sometimes actually creating a whole new monster using an old monster's name! It's an odd set of circumstances, but common enough that I really need to point it out; so here's a list of creatures who've had more than just a starring role!