Amargocephalus

Trampling Step. If the amargocephalus knocks a target prone with its tail, the
amargocephalus can move up to 10 feet towards that creature and make one stomp
attack against it with disadvantage as a bonus action.

Multiattack. The amargocephalus makes two melee attacks. If both attacks hit, the target(s) must succeed on a DC 15 Constitution saving throw or have their initiative number decreased by 2 for 1 minute. 

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 15 (3d6 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 16 Strength saving throw or be knocked prone.

Spiked Hip-Check. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage + 11 (2d10) piercing damage.

Stomp. Melee Weapon Attack: +9 to hit, reach 5 ft., one prone target. Hit: 11 (1d12 + 5) bludgeoning damage.

Invincible Threat Display (Recharge 6). As an action, the amargocephalus can lower its head close to the ground, presenting the spines across its body to attackers. While making its
threat display, the amargocephalus has disadvantage on attack rolls made to attack with
its tail, but gains a +5 bonus to AC. Additionally, whenever a creature touches the
amargocephalus or hits it with a melee attack while within 5 feet of it that creature takes
3 (1d6) piercing damage. The amargocephalus can end its threat display as a bonus
action.

Lesser Counterstrike. In response to being hit by an attack, the amargocephalus makes a melee attack with disadvantage against a target within reach.