If you are at a higher level and you have a longneck rifle expecially one with more than 100% damage use it from a little ways away and you can kill them easily. Then walk up to them and harvest them.
If you dont have the longneck rifle i suggest dont go near them, but if you are a savage and want to kill one take your pike or spear hit it then run. Then turn around with any ranged weapon and just keep shooting. If you dont have a ranged weapon i suggest you not trying to kill one because a few hits and you are going to die.
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.