Y'know the green resurrection obelisk? Yea, they can be found there and can be easily tamed there. Bring a flying mount with decent HP and lure the tickle chicken to a cliff. Demount and spam darts - these chickens can't climb/fly, so that should be the easiest way (other than the travel time) to tame a therizinosaurus. Don't forget nacrotics!

More Therizinosaurus Taming & KO Tips