Multiattack. The tyrannosaurus makes two attacks: one with its bite and one slam attack with its tail. It can't make both attacks against the same target.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 33 (4d10 + 7) piercing damage. The target hit needs to make a CON saving throw(escape DC 14) or be infected. If the target is a Medium or smaller creature, it is poisoned (escape DC 17). Infected creatures take a -1 penalty to all rolls. Infected creatures can make another Con. saving throw at the end of their turn to shrug off poisoned or take 2 damage at the start of each of their turn. Infection can only be removed with a long rest and another creature making a successful DC 15 Medicine check.

Slam. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage. Target needs to make a DC 17 strength check or be pushed back 10 ft. and fall prone. Z-Rex can make this attach as reckless to gain advantage on the hit, but the next melee attack on it will also be with an advantage.