Who Are the Aliens in Marvel's The Avengers?

The fans on the SuperHeroHype Forums made these animated gifs using the new "Head Count" TV spot for Marvel's The Avengers to give us a good and repeated look at "Loki's Army" in the anticipated May 4 release.

Who do you think these aliens are? Have we seen them before in the comics? Are they something new created for the movie? Are they Michael Bay's Ninja Turtles? Let us know what you think in the comments!