I can't remember the title of a late 80's early 90's monster movie. It was about a boy who had a monster that I believe was a stuffed animal which came to life with a really long mouth. This is driving me crazy

I think you're thinking of My Pet Monster (http://www.imdb.com/title/tt0213361/)

I am also looking for somethingblike this does the boy go under his duvet into a world of monsters?

Little Monsters (1989)