Ancient Chultan T-Rex

Huge Size. The T-rex takes up 4 squares. Creatures of medium size or smaller can occupy space unde the T-Rex.  

Thick Hide. The T-Rex has a thick hide that is difficult to break. The T-Rex has an AC 22 against all non magical attacks that do bludgeoning/piercing/slashing damage or are not specialized for killing dinosaurs.

Trample. 

Multiattack. The T-Rex makes one attack with its bite and one with its tail. 

Bite. The T-Rex targets one creature with a Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 33 (4d12+7) piercing damage. On the T-Rex' bite attack it can choose to grapple a creature on hit. The grappled creature must make a DC 17 DEX saving throw. While the creature is grappled it is restrained and takes 33 (4d12+7) piercing damage at the start of the T-Rex' next turn. The T-Rex cannot bite another creature while it grapples. A creature can repeate this saving throw at the start of each of its turns. 

Tail Slam/Tail Sweep. The T-Rex targets a creature with a Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 20 (3d8+7) bludgeoning damage. This creature must be behind the T-Rex or it has to expend 10 feet of movement to turn its large body. Sweeping Attack. The T-Rex' tail attack roll also goes agaisnt the AC of one extra creature within 5 feet of the target provided that creature is no more than 10 feet away from the T-Rex. On hit this creature(s) takes 13(2d8+5) bludgeoning damage.

Meat Toss. While grappling a creature the T-Rex can use its bonus action to throw the creature in a random direction. When a thrown target strikes an object, such as a wall, floor, tree, etc. the target takes 4(2d4) bludgeoning damage for every 10 feet it was thrown (10 feet for ever 6 the creature rolled below the DC 18 DEX save; min 10 feet max 30 feet). If the target is thrown at another creature, that creature must succeed on a DC 14 Dexterity saving throw or take the same damage and be knocked prone. If the saving throw is successful, the target takes half the bludgeoning damage and isn't knocked prone. 

Flank Resistance. When the T-Rex is flanked by one or more creatures it reacts by sweeping its tail. All creatures in 10 foot radius semi-circle behind the T-Rex must make a DC 17 DEX saving throw. On a save the creature dodges the tail and takes no damage. On a fail the creature takes 20 (3d8+7) bludgeoning damage, is pushed out of a flanking position, and is knocked prone. 

Stomp. The T-Rex can stomp whenever a creature tries to run under it. All creatures in the T-Rex' space must make a DC 16 DEX saving throw or take 16 (4d6+4) bludgeoning damage. Creatures that fail this save have a chance to become trapped under the T-Rex' (5% for every 4 below the DC 16 DEX save; min. 5% max 20%). A trapped creature is incapacitated until the end of their next turn. 

Round 1 Roar! . The T-Rex lets out a vicous roar. All creatures within 300 feet must make a DC 15 WIS saving throw or be frightened of the T-Rex. Any creatures within a 20 foot semi-circle in front of the T-Rex must make a DC 18 WIS saving throw or become Paralyzed with fear. A creature can repeate this saving throw at the start of each of its turns.

Round 2 Charge. The T-Rex violently charges in a strait line at a creature, prioritizing creatures frightened or running from it. All creatures medium or smaller the T-Rex passes over must make a DC 16 DEX saving throw or take 16 (4d6+4) bludgeoning damage. All creatures large or bigger must make a DC 16 DEX saving throw or take 16 (2d8+8) bludgeoning damage and be knocked to the side 5 feet. The T-Rex stops on the opposite side of the target creature and turns to face them. Creatures that pass the DEX save can make an opportunity attack if they choose to.

At zero hitpoints Death Throws. In its last breaths the T-Rex vomits bile in a 30 foot cone. Creatures in the cone need to make a DC 16 DEX saving throw or be covered with bile. On a failed DEX save, creatures need to make a DC 14 CON save or take 13 (3d6+4) acid damage. Any creature covered in bile attracts jungle predators which gain advantage on survival checks to track them. Four pints or flasks of water are needed to remove the smell.