The rulebook says that an attack of opportunity can be done on a foe leaving your reach area without disengaging. My thought is kind of reminiscent of the old wording "leaving a threatened area", but anyways, here it is :
The "hooves" attack action description is as follows :
Hooves: Melee Weapon Attack: +5 to hit, reach 5 ft. one prone creature. Hit: 8 (2d4 + 3) bludgeoning damage.
(emphasis added; quote slightly modified for clarity)
Technically, could the hooves attack range be considered 5ft-ground, causing a standing-up character withing 5ft of an Elk's hooves to trigger an attack of opportunity?
Note: This question is not a duplicate. It is specific to the Hooves attack which has very limited usage. The target needs to be proned and within 5 feet. This question is not about "how to perform an attack of opportunity", but rather IF one can be performed with a hooves attack under certain conditions.