Lystrosaurus

Headbutt. If the lystrosaurus moves at least 10 feet straight toward a creature and then hits it with a ram attack on the same turn, the target must succeed on a DC 13 Strength saving throw or be knocked prone. If the target is prone, the lystrosaurus can make one bite attack against it immediately as a bonus action.

Ram. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 3) bludgeoning damage.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 3) piercing damage.