Adult T-Rex

Highetened Scenes: The T-rex has advantage on perception checks that involve sight hearing, or smell
Savage Assault: If the T-Rex moves at least 20 ft in a single turn towards a target and hits with a slam attack the creature must make a Dc 20 Str save or be knocked prone. If a creature is knocked prone the T-Rex can make a Crushing stomp attack as a reaction.

Multiattack. The tyrannosaurus makes two attacks: one with its bite and one with its tail.. It also gets to use its Daunting Roar if it is able to be used.
Bite: Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 47 (6d12+8) piercing damage. When you attack a large or smaller creature it must make a DC str save or become grappled. When grappled in this way the creature counts as restrained
Slam: Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 35 (5d10+8) Bludgeoning damage
Tail Swipe: The T-Rex sweeps its tail in a wide arch behind it, any creature that is behind it must make a DC 20 Str, or Dex save.On a failure the creature takes 5d12 Damage and is either knocked proline(dex) or thrown 20ft away(Str)  (A success takes half damage, and has no negative effect)

Roar of the Colossus(Recharge 5-6): The T-Rex lets out a loud roar, all creatures must make a Dc 18 Constitution saving throw or become defined and frightened of the t-Rex. On a failed save creatures take 12d8 thunder damage, or half on a successful.

Crushing Stomp: If there is a creature within 5ft that is prone you can make a Stomp attack as a bonus action.: +12 to hit, reach 5 ft., one target. Hit: 26 (4d8+8) Bludgeoning damage. Any creatures medium or smaller in a 15ft radius must make a Dc 20 dex save or be knocked prone.
Crushing Jaws: If a creature is held in your jaws as a bonus action you can make a “Crushing Jaws” attack as a bonus action. +12 to hit, reach 5 ft., one target. Hit: 26 (4d8+8) Piercing damage.