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 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.
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.