If you're lucky, you can find one of these alone. I would recommend targeting one that is alone, unless it is too high of a level or too low. Therefore you don't have to worry about Carnotaurus trying to kill you. Kill off anything ot might try to pick a fight with, like mammoths (you can use the prime meat from them to tame it). It takes a good amount of arrows so best if you use tranq darts instead. Just be careful of its roaring because there could be Carnos nearby.
It really is that simple though, they can't reach you or your flier on a big enough rock, so if you don't want to make a trap you can just shoot down from a rock or cliff once the carnos are dealt with. Be ready for a goose chase once it starts running.
If you have Desmodus, attach to a wall and fire one dart. The UT will fear roar but you won't detach and you can wait it out. After, just fire away until he runs, then detach from wall and chase and shoot. Took me about 15 minutes of chasing but he didn't fear roar after he started running. Used this after traps failed me
Easiest way to tame one: go find one(I found a lot near the blue obelisk), pick off carno's with argy. Place behemoth gateway and gate, open gate, behind behemoth gateway place another behemoth gateway without gate this time.
Place a third behemoth gateway with a gate closed behind the one you just placed. (Make sure behemoth gateways are close to each other, I mean... CLOSE!) Place large beartrap inside the trap, then lead it into trap. Close gate and tranq. When done tranqing wait for it to starve (bring narcotics just incase) last step... feed it. Now it is yours. (Try to bring an empty cryopod)