Amphicoelias

Massive.  The amphicoelias cannot be pushed or knocked prone by a Large or smaller creature. 

Stomp.  Melee Weapon Attack: +18 to hit, reach 20 ft., one Large or smaller target.  Hit: The target must make a DC 26 Constitution saving throw.  On a success, the target takes 46 (8d8 + 10) bludgeoning damage.  On a failure, the target dies. 
Tail Lash.  The amphicoelias lashes its tail.  Each Large or larger and not Huge or smaller and prone creature behind it and within 125 feet of it must make a DC 26 Dexterity saving throw.  On a failure, a target takes 34 (7d6 + 10) bludgeoning damage, is knocked prone if it is Huge or smaller, and is knocked back 10 feet for each 5 points of damage dealt if it is Huge or smaller.