Easiest and safest way to tame this is to lure it under water. They stop aggro and you can tame as you please. Just make sure you come up for air every once in a while. Also be sure the water is deep enough.
Full Ghillisuit, bug repellent, and balls of steel because these are scary to tame! They have the taming slot right behind their head about where the saddle would normally sit. Must approach from behind and only approach when feeding, stay away otherwise
I will make a list of pros and cons about this creature:
Nothing, and I mean NOTHING will aggro to you, even if you attack it
Can melt armor, harvest dead bodies from afar with acid ball
Swims the same speed as walk
Terrible weight (100) you're going to have to stuff your whole inventory somewhere, and maybe even your armor
Saddle requires 35 obsidian
Something I've noticed:
Wild ones can deal constant (like .5 sec between attacks) damage to you, but tamed ones, not even mounted ones can, and I get this needs to be balanced, but make it only work on wild creatures or something, I would love to see it work
On the artifact cave on the island near rock island. ( Artifact of the hunter) they spawn on land and in water. When they spawn in water you can go in the water and tame them they are passive in the water.