Disclaimer: if you are on any other platform besides old mobile then ignore this tip.

On old mobile these things are neutral to all tames, meaning that if you try to lure a wild rex to a rando's base to be evil and they have all their tames set to passive, then that's not gonna do anything unless you manually aggro the rex onto you then tank each hit with your tame and the dude's tames in the rex's hitbox until something dies.

This also means that if a rex is near your base it won't be a huge problem until it is practically walking on your tames and your structures, in which you will have to kill it before it gets aggroed on you then starts attacking things in its way.

This also applies to gigas

However if you try this on any other platform including ark use you and your tame will get chomped up as wild rexes are aggressive to all tames.