to tame these beasts, make sure to bring a ton of mejoberries (or crops and kibble) and bio toxin/ narcotics. get on a rock where it cant reach you and shoot it down and chase it when it starts running. once its knocked out, feed it your taming food and make sure to keep an eye on its torpor and feed it bio toxins or narcotics regularly. it may take a while but be patient and sooner or later you will be owning your very own therizinosaurus