If you are going to go hunting with it, it’s best to just wait until you are on the right level and get enough materials to make a saddle and ride it. If you can’t wait till then, definitely don’t hunt near water or the ocean at least. On my first day of my first Rex, I got carried away, got on my Carno, and started to attack a Brontosaurus. It retreated into the ocean and my Rex chased after it, then a surprisingly big swarm of Megs attacked my Rex and I couldn’t save without getting out alive. It managed to kill a few Megs, but it wasn’t a match for all of them. My point is, if you want your Rex to live, think before you are about to do something with it.