Before you can enter the door with a Snake Head, you must discover the three hints. To find them, grab and throw an enemy into the mouth of the Living Tree (big tree with a face). There are three Living Trees in the Living Forest. The first is before the statue that you can save on. The second is beside the door on the map where you can find Reptile's Yingyang. The third is on the right side of the door with the Snake Head.