I dont know why the dododex consider dunkles torpor drain as high, just keep hitting it with tranq just like regular tame, it drops pretty slow compared to other tames that classified as high torpor drain. You didn't even need narcotics to tame it. But maybe it classifies as high torp whenever you miss your tranq arrow, because it is dropping fast.
Very easy to tame if you know what your doing. First tame an Ichthyosaur. Then grab a crossbow and some tranq arrows and find one and knock it out. It is not be able to attack if you are on the Ichthy. As you are taming it, be sure to remain on the Ichthy unless feeding narcs, so other creature don’t hurt you.
Very hard to get it to chase you, so you’ll have to chase it. An ichthyosaurus is best because it’s fast and you can shoot from the back of it. Though, I would recommend bring a basi for protection from eels, megs, and jellies, just to be safe. The dunky is slow but it can go far.
Also, don’t take your eyes off of it for too long or you’ll never find it again.