I have never seen a setting or a game that really used all the creatures in the MM. They were assumed to be there, somewhere, yes, but did you ever see them in an adventure module? Did you actually seriously use one in a game? Have you seen someone else use one?
If you confirm that you have for any of these creatures and prove me wrong I will hate you forever for the rest of the day.
(Editor's Note: I'm going to take some credit for putting the images together for this one, particularly the ones that took me 9,001 hours in MS Paint. All other images copyright Wizards of the Coast, except for when it is not, derp.)