Amargasaurus

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

Stomp. Melee Weapon Attack: +7 to hit, reach 5 ft., one prone creature. Hit: 20 (3d10 + 4) bludgeoning damage.

Tail. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 11 (3d4 + 4) bludgeoning damage plus 4 (1d8) piercing damage. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be knocked prone.

Threat Display. As an action, the amargasaurus can lower its head close to the ground, presenting the spines on the back of its neck to attackers.  While making its threat display, the amargasaurus has disadvantage on attack rolls made to attack with its tail, but gains a +2 bonus to AC. Additionally, whenever a creature touches the amargasaurus or hits it with a melee attack while within 5 feet of it, that creature takes 4 (1d8) piercing damage.  The amargasaurus can end its threat display as a bonus action.