Easy tame.... Just make a 2x3 stone foundation and cover 3 high with doorframes on the sides. And 2 high on either end. Place wooden ramps (They're temporary but easy to make and repair) just get aggro and run inside the trap. Takes about 30-45 tranq arrows from a mastercraft longneck. On mobile the kibble is easy enough to make. Get a fast mount with knockback. Go to fars peak and run around pulmonoscorpions to find their eggs. The kibble needs prime jerky and longrass so if you have that set up, rush a few caves give it a soothing balm and under level 70 only needs one kibble. So that's a convenient benchmark. It's expensive to build the trap big enough to get the red out so once tamed just request it back to base. Unless a Giga or enemy tribe shows up it will be untouched if left alone.
I would think these were gonna be harder to tame but they’re actually not that hard, my stamina and movement speed is pretty high I’d say around 200 or higher but I didn’t need a trap or any of that, all I used was tranq arrows, u shoot it once then run away for a little bit turn around quick shoot it again then run away again and shoot it again and repeat, if it starts running it means it’s almost knocked out just chase after it and keep shooting and feed it meat!
If you use the bear trap strategy which is if you lead it into a Dino gateway that is shaped in a triangle and with a bear trap in the middle (Large Beartrap) and when it is stuck you must put a dinogate behind it to trap it and then knock out.
KOing one and taming isn’t a problem it’s just most of the time where ever you knock out a Rex there will most likely be other predators around that can kill it while unconscious, I would clear out the area or lure it into a different spot if you can see other animals that might interfere with taming.
I recently tamed a Rex and it was not that hard at all. So if the Rex u are looking for is level 1-15, Then you only need like 30 tranquil arrows. Just find a hill bigger than the Rex and can’t be climed by one, Then get its attention and start shooting. Just don’t let anything eat it when it gets knocked.