If you are on an animal it's way easy to tame these guys. I'd also recommend having the kibble and having a creature of any kind with high health. You can knock these guys out on foot too, but it's a lot harder and you risk other animals killing you/the rex. I'd also recommend not going after any mate boosted rex, since the torpor you do is directly related to the damage you do.
Dont be scared of them, get a flyer and make sure to have a high health and stamina, fly at him (make sure its only one or your dunso) and let him take a bite of your flyer once in a while then lead him into a square wall of dino gates with a trap in it. Once he is stuck quickly close up the open gate, laugh at him and shoot him (alot)
How I tamed my Rex (tyranitar) I went to the volcano and got him stuck on a bump I shot him from there but first I tried to bear trap him but he broke out quickly my Argy nearly died but thanks to a saber tooth we got away I came back and start lurring him to another trap butmissedand got stuck and i tamed him now he keeps my diploclaus alive when sarco attack