For ark mobile just have things to build a pen around it when it is knocked out and request it (to request dinos got to the player menu and there should be a button that says “tames” above your stats, press that and tap a Dino’s name and a button that has “request” will be able to be pressed you will not miss the request button, also requesting dinos tp’s them to the bed that you/ your tribe own that is CLOSEST TO THE DINO NOT THE PLAYER)
These guys are hard to find because they sink into the mesh underwater. I recommend flying over the swamp with an argy looking for any signs of movement or sails. Sometimes they’ll be on the shores of the swamp, too, so check there.
Once you find one, pick it up (if you’re on an argy) and take it to a safer place. Otherwise, just kill any dangers in the area and chase it around with your tranq method of choice. Use their huge turning radius to avoid their bites.
Once tamed, level melee to increase their insulation. Good luck!