The best way to avoid looking humanoid is to not have a body at all. Energy aliens are non-humanoid aliens that exist as vibrational frequencies of incorporeal energy. The best, recent example is in this year's The Darkest Hour, but it has a long and storied history. The first appearance of the concept comes in H.P. Lovecraft's The Color Out Of Space, where a bodiless creature crash-lands to Earth in a meteor and drains the life from everything around it. Interestingly enough, the Metroid aliens in Metroid, even though they look like giant parasitic jellyfish, are actually composed of pure energy as well.
