Do NOT trap this dino. It will usually (not always but usually) aggro to the trap and when it stops it will nerf out and won't eat. It will sniff but not eat.

Get a very durable wyvern with lots of health, set it to follow you, let it munch on the wyv, throw out a baby rex, kill it and drag to carchar. Once carchar sniffs and eats you should be able to ride instantly. Repeat till tamed