If you see one, and are trying to fight or knock it out, i recommend going to a ledge above the rex, which in where it spawns can happen, make sure to use a good level longneck or other powerful tool if you want to kill one, and relocate if it gets close (they are not too fast). To knock one i recommend tranq arrows or darts (arrows if you have a good crossbow, they cheaper) and if you knock it just feed it prime meat or mutton, wait and then you get a Rex. If you have a tame that can outrun it you could relocate much easier