Ankylosaur

Armored Defense. The ankylosaurus' thick hide grants it resistance to concussion damage.

Collision Reaction. If an enemy enters its space while the ankylosaurus is in range, the ankylosaurus can make a tail attack as a reaction. This attack has advantage if the enemy has not been attacked by the ankylosaurus that turn.

Tail Attack. Melee Bludgeoning Weapon: +9 to hit, reach 5 ft., one target. Hit: 17 (2d10 + 6) bludgeoning damage. Target must make a Strength saving throw (DC 15) or be knocked to the ground.

Horn Strike. Melee Bludgeoning Weapon: +8 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 6) piercing damage.

.

Defiant Roar (Requires proximity to enemies)
Bonus Action
The Ankylosaurus lets out a loud roar that echoes through the air. Each enemy within 10 feet must make a Wisdom saving throw (DC 13) or suffer the frightened condition until the end of its next turn.

Use: 1 time per short or long rest

Defensive Preparation (Requires standing still or little movement)
Bonus Action
The Ankylosaurus assumes a defensive stance, increasing its resistance to attacks for a brief moment. Until the end of its next turn, it has advantage on Dexterity saving throws and its enemies have disadvantage on rolls to attack it with ranged weapons.

Use: 1 time per short or long rest

Quick Reflexes (Requires being attacked)
Bonus Action
The Ankylosaurus makes a small movement with its tail or body to avoid an attack directed at it. The Ankylosaurus can use its reaction immediately after an enemy makes an attack against it, allowing it to move 5 feet without provoking attacks of opportunity and, if the enemy is within range, make an attack with its tail.

Use: 1 time per short or long rest.

Tremors (Requires standing on solid ground)
Bonus Action
The Ankylosaurus uses its mass and weight to cause the ground to tremble around it, causing instability. All enemies within 5 feet of it must make a Dexterity saving throw (DC 14) or fall to the ground. Enemies that succeed on the save remain standing.

Use: 1 time per short or long rest.

Defensive Tail Strike
Reaction
When an enemy enters its space or makes a melee attack against it, the Ankylosaurus can use its reaction to strike the enemy with its tail.
Tail Attack: Melee Concussion Weapon: +9 to hit, reach 5 ft., one target. Hit: 17 (2d10 + 6) concussion damage. Target must make a Strength saving throw (DC 15) or be knocked off balance.

This strike takes advantage of the reach of the Ankylosaurus' tail and gives it the ability to destabilize its attackers.

Reinforced Defense
Reaction
When the Ankylosaurus is attacked by an enemy within 5 feet of it, it can use its reaction to put its "Armored Defense" to use. As a result, the Ankylosaurus' AC increases by +2 for that particular attack, reflecting the natural armor of its thick hide.
Note: This reaction can only be used once per turn.

This resource reinforces the Ankylosaurus's resilience, allowing it to better withstand close-range attacks.

Collision Reaction
Reaction
When an enemy within 5 feet of the Ankylosaurus makes an attack, the Ankylosaurus can use its reaction to lunge at the enemy with a quick swipe of its tail. If the attack is made before the Ankylosaurus is hit, it can use the reaction to knock the enemy down.
Tail Attack: Melee Concussion Weapon: +9 to hit, reach 5 feet, one target. Hit: 17 (2d10 + 6) points of damage.
The target must make a Dexterity saving throw (DC 14) or fall to the ground.

This reaction allows the Ankylosaurus to use its aggression to punish enemies that attack it.

Collision Tremors
Reaction
When the Ankylosaurus is hit by an attack from an enemy within 10 feet of it, it may cause a small tremor in the ground. All enemies within 5 feet of the Ankylosaurus must make a Dexterity saving throw (DC 15) or take 10 (3d6) bludgeoning damage and be knocked back.
Use: Only once per short or long rest.

This type of reaction gives the Ankylosaurus a chance to not only defend itself, but also aggressively damage nearby attackers.