Stock Monster Symbolism

You wanna make a serious minded monster movie? Well, the result is going to be taken as symbolism, regardless of what you intended.

Here, then, are some of the standard Monsters from Our Monsters Are Different, with notable examples of said symbolism as interpreted.

Vampires

 * Anything to do with parasitism, including:
 * Aristocrats Are Evil
 * Dracula
 * Near Dark is interesting, in that it can be taken as being a supernatural, modern, take on the Outlaw Western.
 * Who Wants to Live Forever?
 * Too many Vampire Detective Series to mention
 * Vampires Are Sex Gods
 * Bloodthirst as a metaphor for drug addiction

Werewolves

 * The Beast Within
 * The Wolfman
 * Just about every other werewolf using the whole amnesia shtick.
 * Metaphor for STDs
 * Harry Potter (according to Word of God, although the evil werewolf Fenrir Greyback gives off hints of Pedophilia as well).

Zombies (of the Zombie Apocalypse variety)

 * Humans Are Bastards
 * Just about every Zombie Apocalypse film ever, although oddly, the zombies aren't the source of this symbolism; the reaction of the Humans is.
 * Survival of the Fittest
 * Faceless conformity and loss of identity

The Body Snatchers and similar

 * Conformity and infiltration are the watchwords here.
 * Frequently associated with a Red Scare

Giant Monsters

 * Disasters. Either man made (Hedorah, giant robots) or natural (Rodan), a sort of divine retribution tied to shinto beliefs can also be read into it (Mothra).
 * Godzilla started out as a metaphor for the devastation of the atomic bombs dropped on Hiroshima and Nagasaki.
 * Anti-military aesops. Almost always military might will fail miserably and just waste everyone's time, and often it's the fault of the military that the Kaiju are there in the first place.

Alien Invaders

 * Human imperialism and war.
 * The original War of the Worlds novel was largely meant as a critique of the British Empire and imperialism in general.
 * Human reaction to disaster, similar to zombie films in that the people are more of a problem than the invaders. Also an element of all War Of The Worlds adaptations.

Mad Scientist-made Monsters

 * Science Is Bad / My God, What Have I Done?
 * Turned Against Their Masters