ComicBookMovie as the possible identity of the monstrous creature.
Foom is best known as the giant dragon-like villain of Iron Man. Hulk and Thor have also gone up against him in the comics. He was briefly spotted in Iron Man 2 on a billboard as an Easter egg. It's unlikely that we would see the character in Iron Man 3 since there's been talk that future films will be more grounded. Avengers on the other hand is a perfect place to bring in creatures like Foom into the picture. Since he could be called out of the other realms, outer space or with magic pulled out of the billboard into reality by Loki. While I think Avengers is a great film to use Foom Marvel Studios' isn't known for overloading their movies with villains. I also don't think Whedon is looking to have his own "Release The Kraken!" moment.