Five Big Snakes spawn in a cave on Crash Island. The pit they are in is a multicombat zone, so it's recommended to use Protect from Melee prayer. If you are too close to the walls, the snakes popping in and out of holes can hurt you as well. Its guaranteed drop, the snake corpse, is cooked during Recipe for Disaster. However, its poison does only damage you by 1 HP.
If for some reason you kill all the snakes and they don't respawn, just exit via the rope and come back in. This resets the snakes' summons, so if you were killing a snake and accidentally left, its hitpoints restore to normal and you must start anew.