Will a dragon fall if it drops prone in flight?

Question

If a dragon decides to drop prone while flying, will it fall?

To meet the minimum character requirements, here are the rules for Prone and Flying Movement:

Combatants often find themselves lying on the ground, either because they are knocked down or because they throw themselves down. In the game, they are prone. You can drop prone without using any of your speed. Standing up takes more effort; doing so costs an amount of movement equal to half your speed. For example, if your speed is 30 feet, you must spend 15 feet of movement to stand up. You can't stand up if you don't have enough movement left or if your speed is 0.

To move while prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot. Crawling 1 foot in difficult terrain, therefore, costs 3 feet of movement.

Flying creatures enjoy many benefits of mobility, but they must also deal with the danger of falling. If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.

Answer 1

Short answer: Yes

While there is a difference between dropping prone and being knocked prone, the free action of dropping prone causes the creature to

throw themselves down.

In this situation, the dragon is choosing to voluntarily fall. In the related question, the accepted answer gives you some reasoning as to why that may be; for example, if you choose to use the 500 ft/turn rule, and the dragon seeks to dive bomb or escape, moving well over 500ft in a single turn using prone plus the dash action. This works as well with basic rules, but is a bigger risk to the dragon unless they can resist the <= 20d6 bludgeoning, or have the health to walk it off, as well as subject themselves to the prone condition, at least temporarily.

Answer 2

If you become prone for any reason while flying, you fall.

You quoted the relevant text in your question:

Combatants often find themselves lying on the ground, either because they are knocked down or because they throw themselves down. In the game, they are prone.

Being prone is "lying on the ground", so it's not really something you can do while also flying, i.e. not touching the ground. The rules address this by specifying how the interaction of those two states works: you fall. There is no particular difference between being "knocked prone" versus "throwing yourself down". You're dropping to the ground quickly in either case.

Conditions are not arbitrary rules elements that float in a purely conceptual space. "Prone" is not a binary flag you can set on the character that just exists divorced from the game world at large. It represents a physical state that's happened to the character.

Answer 3

A big assumption

Your question assumes that a dragon can choose to go prone while flying. I am not sure that is the case (emphasis and explication mine):

Combatants often find themselves lying on the ground, either because they are knocked down or because they throw themselves down. In the game, they [that is, those combatants who find themselves lying on the ground] are prone.

This passage could be interpreted to mean that throwing oneself down to become prone requires that one end by lying on the ground, and as such, cannot be done while still airborne.

Looking at the rules for flying movement,

Flying creatures enjoy many benefits of mobility, but they must also deal with the danger of falling. If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.

While these rules establish that a flying creature may be knocked prone, they do not establish that a flying creature can choose to 'throw themselves down' and go prone.

Thus it is a rather big assumption on your part that a flying creature can voluntarily go prone, when in fact that merits its own stacks question. However, for the sake of answering this question, let's assume that your dragon can indeed go prone while flying without having to lay on the ground.

Will it fall?

As quoted above, there are three situations in which a flying creature will be forced to fall:
(a) if it is knocked prone
(b) if its speed is reduced to 0
(c) if it is 'otherwise' deprived of the ability to move (here 'otherwise' is ambiguous, but since being knocked prone does not deprive one of the ability to move, we can understand this to mean being deprived of the ability to move in any way other than having one's speed reduced to zero)

Since choosing to go prone fulfils none of these conditions, a creature that chooses to go prone while flying (if permitted to do so) will not fall.

If it doesn't fall, what happens RAW?

Suppose we now have a prone, still-flying, not-falling creature.

First, they should have all the mechanical effects of the prone condition:

A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
The creature has disadvantage on attack rolls.
An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.

Further,

To move while prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot.

So we have a still-flying creature that cannot move as fast but is harder to hit at a distance, and that is easier to hit within 5 feet but which has a harder time itself hitting.

What I would do

How should we represent the rules consequences of flying while prone within the narrative of the game?

Being knocked prone is one thing, but choosing to go prone typically occurs when a character is protecting themselves from missile fire. To me, it makes the most narrative sense to leave behind the description of prone as being 'crawling on the ground' and instead assume that the flying creature is taking evasive maneuvers; rolls, high-speed turns, sudden changes of trajectory, and such. These actions would be under their own control, would reduce the net distance they traveled, would make them harder to hit unless their attacker managed to get in close, and would make it more difficult for them to attack, neatly fitting all of the mechanical effects of being prone while flying.

Thus, narratively, I would assume that:
The prone condition on the ground means that you are crawling
Being knocked prone while flying means that you are falling
Choosing to go prone while flying means that you are making evasive maneuvers

This is my opinion; other narrative interpretations may apply in other games. However, the mechanical consequences of being prone while flying (assuming one can choose to do so) are RAW and must apply regardless.