What is the name of this Anime?

I vaguely remember this, so hopefully someone knows. It was on Adult Swim around 6 or 7 years ago. These aliens/creatures are in a forest fighting others, and they can shoot laser beams out their forehead. At the end of the episode, one of their head (beheaded) were on the ground. Sounds kinda weird. It has been stuck in my head today and I'm curious of what its called. Thanks.