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.
Best Answer
No.
The reach is 5ft and can only hit 1 prone creature. The reach is not "5ft on the ground only." For example, you could target a prone creature laying on top of a low table. They do not need to be on the ground - just prone within 5 feet.
The mechanic of standing up from prone requires you to use your movement feet, but does not actually entail you moving anywhere. The feet used to stand is representative of the time you need to recover, not the distance traveled. A human does not move 15 feet when they stand up. Standing up does not trigger opportunity attacks.
Even by the old wording you are not leaving a threatened AREA, you are leaving a "condition"(at best) that could be exploited by hooves.
The argument could be made an attack of opportunity is possible while a prone creature is crawling away without standing up.