I killed everything looking for this egg! Until I saw this post from Allah: "Uglord is right about the best spot to camp the 'sinuous adder'. He's the only one that drops the egg, and if you follow the south wall to the east side of the river, go north just a little bit following the river, stand there looking north and you will see 3 roamers pathing right in front of you. One of them is always the sinuous adder."