The Giant Elk entry says:
Charge. If the elk moves at least 20 feet straight toward a target and then hits it with a ram attack on the same turn, the target takes an extra 7 (2d6) damage. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be knocked prone.
Then in the attacks block, it states (bold for emphasis):
Hooves. Melee Weapon Attack: +6 to hit, reach 5 ft., one prone creature. Hit: 22 (4d8 + 4) bludgeoning damage.
So does the Elk receive a bonus Hooves attack vs a foe it just knocked prone? If so, would it receive this bonus attack against a prone foe every round it remained prone? One assumes that if the foe is not prone, the Elk would not get this attack at all with the way it is written.