Mammoth

If the mammoth moves at least 20 feet straight toward a creature and then hits it with a gore attack on the same turn, that target must succeed on a DC 18 Constitution saving throw or be knocked prone.
If the target is prone, the mammoth can make one stomp attack against it as a bonus action.

Melee Weapon Attack: +10 to hit, reach 10 ft.. Hit: 25 (4d8 + 7) piercing damage.

Melee Weapon Attack: +10 to hit, 5 ft., one prone creature. Hit: 29 (4d10 + 7) bludgeoning damage.