Extremely easy to knock out and tame. As soon as you have a tranq arrows and hide Armor unlocked, go to Carno Island at the top right of the map and stand on a cliff. Hide on a cliff and kill regular beasts until you get a rex, stay on the cliff and shoot it repeatedly. It will fall. You can feed it Raw Prime meat and it will tame very quickly. Raw Prime meat you can get fron the argys and carnos on the island. You shouldn't need a lot of narcoberries for a low level one. Got 3 rexes this way. Before I even tamed a single carno.