Suppose, if this triceratops shall move at least 20ft., straight towards the creature and after then hits it with a gore attack on same turn, basically, that target should be succeeded on a DC13 Strength saving throw or else be knocked prone. However if a target is prone, the triceratops could make a stomp attack which is against it is a Bonus Action.
Gore: Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 24 (4d8 + 6) piercing damage.
Stomp: Melee Weapon Attack: +9 to hit, reach 5 ft., one prone creature. Hit: 22 (3d10 + 6) bludgeoning damage
Attributes Of Triceratops DnD 5E Monster
|AC||13 (Natural Armor)|
|Roll 0||Gore 1d20 + 9 4d8+6|
|Roll 1||Stomp 1d20 + 9 3d10+6|