One popular theory is that the face really looks like Beta Ray Bill, which has some people suggesting the Korbinites. However, the Korbinites were light orange, while the Avengers aliens have more of a greenish hue. Plus, Beta Ray Bill has a unique look, and the other members of the Korbinite race have more of a humanoid look. Another popular theory is the Badoons. The Avengers aliens greenish skin color is a good match for the Badoons. The face is a little different than how Badoons have typically been portrayed in comics, but maybe some liberties were taken for the theatrical version. There are also some other features that appear to match like the pointy toes and the pointy ears.
One other theory suggests that the alien race is somehow tied to Thanos. There have been rumors for awhile that Thanos is a mystery villain in The Avengers. While the face of this alien doesn’t look anything like Thanos and the skin color is wrong, the helmet is a dead ringer for Thanos.
Between these three theories, they all seem to be close, but just not quite right, which led us to come up with the following theory. What if Beta Ray Bill mated with the Badoons and dressed his offspring like Thanos? It’s the only answer that makes sense at this point. Avengers mystery solved.