Clive Barker recently suggested "Monsters act out our rage. They act on their worst impulses, which is appealing to a certain part of us. They get punished for it, but we've enjoyed the spectacle of their liberation."
For many years, I've taken Ishiro Honda's view on monsters, where he suggests "Monsters are tragic beings; they are born too tall, too strong, too heavy, they are not evil by choice. That is their tragedy." Both views are somewhat incomplete, but an interesting point from which we might create interesting horror figures.
A noteworthy perspective was Stephen King's, who once said "Monsters are real, and ghosts are real too. They live inside us, and sometimes, they win." More classically, Victor Hugo suggested "Adversity makes men, and prosperity makes monsters."
Since we'll be looking at zombies significantly in 2013 thanks to Saymoukda Vongsay's Kung Fu Zombies vs. Cannibals, it's important to bear in mind George Romero's quote: "I also have always liked the monster within idea. I like the zombies being us. Zombies are the blue-collar monsters."
Francisco de Goya weighed in with the notion that "Fantasy, abandoned by reason, produces impossible monsters; united with it, she is the mother of the arts and the origin of marvels."
The wonderful thing about monsters is that there really is no last word on them in any generation. The very best monsters show us something about ourselves and about the world. These beings at their best return generation after generation but always find a way to speak to us, bridging both the past and the present.