Can an Elk make an attack of opportunity with its Hooves when an adjacent creature stands up from being prone?

Question

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.

Answer 1

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.

Answer 2

Opportunity attacks happen when the opponent leaves your reach

Standing up doesn’t do this. Nor does running around you in circles within 5-feet. The target must move from within 5-feet of you to not within 5-feet of you.

If they are prone while they do that (i.e. they crawl) you can use Hooves.

Answer 3

You do not move when you stand from prone and thus standing from prone cannot provoke an opportunity attack

The rules on "Opportunity Attacks" state the following:

[...] You can make an opportunity attack when a hostile creature that you can see moves out of your reach [...]

There already exists a similar question here:

• "Does standing up from prone trigger the damage from Booming Blade?"

The answers there establish that booming blade triggers its damage when the target moves and that the damage does not trigger when a creature stands up. Thus, standing up does not count as moving and cannot provoke an opportunity attack.

The answers there are well-accepted but the following unofficial ruling (a tweet) from lead game designer Jeremy Crawford used in BloodCinder's answer also exists:

Question: For Booming Blade, did you intend for standing up from prone to trigger the extra damage? It costs movement.

Crawford's Answer: Standing up costs movement but moves you nowhere. To move while prone, you crawl or use magic (PH, 191).

Note that Crawford's quote is somewhat problematic because teleporting also does not count as moving...

For a more direct answer there also already exists the following question:

• "Does standing up from being prone provoke an attack of opportunity?"