Follow it with wooden spike traps. Place one in front of it, then quickly place the other two around it, encasing it. Don't worry, the spikes won't hurt it. Then place the wooden pillars next to it, along with a few thatch ceilings above it. Go from there. It won't be able to walk or fly away. Just be careful of surrounding Dino's.
They are believed to be part of the Tardigrada phylum, a group of extremophile microgranisims that can survive literally any environment, from extreme cold to extreme radiation, these guys are the hardiest of animals.
They have a damage resistance when inflated, so use a snow owl to freeze and deflate. After latest update, they can no longer be picked up or hit with bola. Heavily suggested to clear area of hostiles before KO. If timed right, you can keep gasbag in same area while tranqing by alternating freezing and shooting.
If inflated it will take 400 % more darts/bullets hits ect. To knock it out / kill it so dododex is not wrong it's just because the gas bag is inflated. Also bring cryppded anky or magmasaur with pick or minding drill to collect a huge amount of metal! While rideing of course because it has a ton of weight.
Just trap them with behemoths and use snow owl to deflate, shoot a tranq, mount the snow owl again and repeat until tamed.
P.d.: And the end, he will start trying to get away and then u can shoot him like 3 times before he inflates again