Pretty easy to tame when you have the correct equipment, but extremely useful creatures. Get yourself a reasonably sized tame hole with 3 high stone walls, ramp on one side, door on the opposite and a dinosaur gate (worked for spino's as well) for the end... If your movement speed is good enough lead it into the tame hole, run out the other side and shoot it with tranq darts until it's down. Keeps you safe without much stress during the entire taming process, and is even better if the hole is close to your base along with the Rex's themselves. We currently have 6 within our tribe... One of the best tames to have imo.