How to tame:
Step 1: Find one (They are found in SE and Rangarok
Step 2: Feed it all your allies and dinos (Let it kill all of your tamed dinos and tribemates)
Congrats! You have tamed a Thorny Dragon!
If you are going to tame one of these with no dinos, be sure to upgrade your movement speed and get tranqs. When you see it whip its tail forwards move to the right you will dodge the shots. This worked for me.
A good thing to do for a lot of creatures is build a cage out of door frames and ramps once the creature is in just shoot it with tranq arrows or darts but the creature has to be big enough to not walk through the door frame
If you have a wyvern or argy, easiest way I found to tame is to set up a taming pen, 2x4 with single door frames on the first level, walls on the second(stone), with a Dino gate on one end. Go pick up your dragon of choice and drop it in! Super easy and quick.