The dossier says:
"Found only in the dark caves of the island"
They are already wrong! It should say something more like:
"Found everywhere but the dark caves of the island, this creature will gladly ruin your day by jumping out and killing you as soon as you even think about going to the swamp. Nearly impossible to tame because it has to be tamed via passive which is very difficult to do. Worst of all you need to feed them fertilized eggs. Why can't it e a little easier?
If you kill it and look in its inventory, its got a lot of good stuff like bows, armor, and arrows or cooking recipes. I use bows and pikes to kill them, and pack stimberries. Im playing on easy-casual on ark mobile so might not be the same for other versions.
You tame them by droping ferti eggs from your inv on the ground. You need to do so when the snake is not aggroed, so sit on a rex or smthing it wont attack. Be sure to drop them from your inventory, not from dino ones, that will make it bugged, it wont eat. To a lvl 20 Rex egg gives 29% on 2x event, bronto 26.7% and giga and wyvern the same, looks like all the big eggs give approxymately the same amount.. so dont waste the best you have ? If they get aggroed on you just run away and return, taming wont drop. Happy taming!