Jyu-Oh-Sei: Planet of the Beast King Juu Ou Sei

Thor and his twin brother, Rai, are cast off to a forsaken planet by unknown masked men after witnessing the suspicious death of their parents. Given only a beam knife and a smelly satchel, they wander together in search of civilization on the primarily plant based world. They soon discover the plants have the advantage. Only the strong survive; the ones who throw away their humanity and become the beasts humans once were.

(Source: Anime News Network)