Personally, I like the idea of a killing meteor (ala Final Fantasy VII) rather than a rampaging monster. Final Fantasy VII did the meteor idea right. The entire planet can see the approaching meteor suspended in the sky several days like a huge star before it impacts. People fall into despair looking at the sky; what can you do about the approaching impact? The PCs will have to undertake some epic quest. I like that approach because stopping the destruction of the world doesn't involve simply killing some super-monster. For example a possible quest could involve getting an artifact that can reverse the path of the meteor (perhaps the same artifact was used by the BBEG to "summon" the meteor).
However, if you want a rampaging beast I would personally sugesst a kaiju-style monster (those Japanese giant monsters). Although "guy in a rubber suit" type B-movies might immediately spring to mind, I think that kaiju style monsters would be very appropriate as a harbinger to the end of the world. If you get over the image of cheesy Japanese B-movies, you realize that some of those monsters are rather freaky looking! With horns and tentacles, they are the perfect bringers of doom. After all, in most fantasy settings dragons, undead hordes, and armies of evil humanoids are hardly enough to freak people out. (Sure they might be powerful, but in a world filled with magic, dragons and undead are hardly unique.) Besides, kaiju-style monsters were used as some of the most powerful oponents in a number of popular video games (such as the Weapons in the Final Fantasy series).
