Charge. If the rhinoceros moves at least 20 feet straight toward a target and then hits it with a gore attack on the same turn, the target takes an extra 3 (1d6) bludgeoning damage. If the target is a creature, it must succeed on a DC 15 Strength saving throw or be knocked prone.

Gore. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 14 (1d6 + 1) bludgeoning damage.

Full Charge. All rhinos gather and charge the party. Each PC (Player Character) must succeed a DEX save or take a d4 of damage for each rhino (can be modified to preference). The rhinos don't have to roll to hit, they only roll for damage on failed saves.