How to tame.

Step 1-make a line of 16 to 20 giant bear traps where you want to tame your Rex.

Step 2-Agro a Rex and get it to step on the first bear trap.

Step 3-shoot with tranqs until knocked out(I suggest a longneck rifle and a lot of tranq darts).

Step 4 -put 20-30 prime meat into its inventory.

Step 5-enjoy your new Rex.