Size matters. if the T-Rex is not an adult it's speed is doubled but it's AC is 13 and it doesn't have strength or constitution proficiencies

 Siege Monster. The Tyrannosaurus Rex deals double damage to objects and structures.

Keen Senses. The Tyrannosaurus Rex has advantage on Wisdom (Perception) checks that rely on smell, sight, or hearing.

Thick Skin. The T-rex is resistant to all nonmagical damage delt with weapons weaker than iron weapons.

Combative Mount. A trained dinosaur is proficient with light, medium, and heavy armor.

Bloody. When this creature takes more than 100 damage in a round it gains the bloodied trait. This trait allows blood based actions to be used (ex:Adrenaline Boost).

Multiattack. The tyrannosaurus makes three attacks, and it can't make more than two attacks against the same target.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 33 (5d10 + 9) piercing damage. 

Full Power Bite. Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit: 30 (7d12 + 10) piercing damage. If the target has an AC lower than 15 the damage doubles every level of AC lower, and this counts as two attacks.

Tail. Melee Weapon Attack: +8 to hit, reach 25 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage.

Rush. Melee Weapon Attack: +7 to hit, reach 40 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage, and the T-Rex moves 40 ft in it's path of attack. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be knocked [condition]prone

Reposition. The T-rex can do a quick movement 10 ft in any direction.

Stomp. Melee Weapon Attack: +9 to hit, reach 10 ft., one prone creature. Hit: 22 (3d10 + 6) bludgeoning damage.

Grab and throw. When a creature approaches within 5 ft of the T-rex it can grab and throw the creature 20 ft in any direction with a DC 15 strength saving throw.

Reposition. The T-rex can do a quick movement 10 ft in any direction.

Adrenaline Boost. When the T-Rex is Bloodied, it gains a boost of adrenaline, this boost makes it negate 3d6 of damage or gain 3d6 damage with 8 charges of resistance or damage per boost.

Terrifying Roar (Recharge 5-6). Each time the T-Rex roars each creature within 120 feet and able to hear the roar must make a saving throw. Each creature that fails a DC 15 Constitution saving throw is frightened for 1 minute. A Frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.