This boss is optional atm, if you go to where you plunge on its head it won't be there unless you've gone through the door behind the Gun Fort shrine. There's a way to spawn the snake from the Senpou Temple, though. You basically jump off the plank to get your health low, use contact poison and wait until you have a couple pixels of health. Then you jump off and die in mid air, allowing you to resurrect on the rock the snake is normally coiled around. Heal up and start jumping off towards the bridge until you spawn the snake. warp back up and you can kill it.