Last week, hubby and I managed to see Pan's Labyrinth. You've probably seen Pan, the bluish creature with the big horns, in all the promos and while clearly they spent big bucks to make him real, the true monster in the film is a human. Whether you like your monsters human or otherworldly, I highly recommend the film; I also wrote a full review for The MediaNook.
... via Drawn! I direct you to the Ray Harryhausen Creature List. Oh, WOW. Not only are the creatures itemized, but clicking on its name brings up a short film clip of the creature in action.
... and while not quite monsters, the creatures in Michael Mills' classic NFB* animated short, Evolution (1971) are not of this earth either. I was happy to watch it in its entirety today on YouTube, though I'm not sure the NFB would be happy to find it there. [While at the NFB, I found a bunch of their short films are available online so I watched another favourite, The Cat Came Back.]
*that would be the National Film Board, well known for producing Oscar®-winning animated short films, among other things.