The easiest way to tame is to bring a high lvl Argie, a spyglass, some food, and a crossbow or longneck with tranqs to carno island. Kill everything near the Rex including all of the lower level rexes, leaving only the highest level Rex. Then, aggro it and fly up to the center piece of the island and find a safe perch to dismount. Shoot it from there, and when it runs away get on the argie, fly closer, land, dismount, and continue shooting until it is too far, then remount and repeat. Once it is tranqued, get some prime from whatever is near, and put it in the Rex. Take your argie and fly it above the Rex, and just keep an eve out for anything that might attack it. Once it’s tamed, cryopod and fly back to the nearest allied base. You can repeat this and get a different gender, and begin building up an army. Fair hunting a good day to you.