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
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.
All you have to do is tame a scorpion and ride up to an arthro on the scorpion and feed it spoiled meat. The arthro will not aggro on you while you are riding the scorpion. I did this on my mobile world so I can confirm that this works at least on mobile.