What happens when you are knocked prone while mounted and make the saving throw to remain mounted?

Question

The rules for mounted combat include a special effect for falling prone (emphasis added):

If an effect moves your mount against its will while you’re on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. If you’re knocked prone while mounted, you must make the same saving throw.

If you are on a mount and you are knocked prone but make the DC 10 Dexterity saving throw, what exactly happens? Are you prone but still mounted? What practical effect does this have, compared to being mounted and not prone?

(Assume the mount is not knocked prone, only you are, for example if you fail the Strength saving throw against a wolf's bite attack.)

Answer 1

You are mounted and prone

In this case, the originating effect is something where the rider now has the prone condition. When mounted and prone, the rider must make another DC 10 DEX save to stay in the saddle.

The rider remains prone (and still mounted) on a success, but falls to the ground and remains prone if failing the DC 10 DEX save.

Mounted and Prone - huh?

In this case, we've got a weird situation to imagine. The rider is prone and suffers all the effects of that condition. It's a simple mechanic with clear effects.

But what isn't clear is how do you narrate being prone while riding a horse.

That bits going to be opinion-based, but this DM would probably describe it as you're still mounted, but you're definitely not seated well and are trying to recover your balance. In this narration, it's being unbalanced that gives you the mechanical effects of being prone and still requires the necessary movement cost to undo the condition.

As long as you keep the mechanics of being prone, you can use whatever narration you'd like to describe what it looks like. But the condition itself remains until resolved.

Standing up while sitting down

The other weird piece to this is that prone is resolved by spending half your movement to 'stand up'. It is unclear whether 'standing up' when mounted means you actually need to dismount first or that 'standing up' is more figurative to just needing to expend movement to no longer be prone.

Because the rules here are unclear, it is going to be up to the DM to determine the resolution for the condition, but this DM would rule that it is the expenditure of movement that resolves it. The act of 'standing up' becomes "sitting up".

Answer 2

You will be prone and mounted.

I don't know of any rules that would prevent you from being both prone and mounted. And then the wording of the rules is pretty clear. You were knocked prone but you made the save, so you stay on the horse.

What practical effect does this have, compared to being mounted and not prone?

This comes for you with all the effects of being prone.

• 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.

I also assume would still stay prone should you dismount, because dismounting is not an option to end the prone condition.

(I think you could also do it on purpose, for example to dodge a hail of arrows. Lie flat on your mount and they are less likely to hit you, just like the films. But this is out of scope for the question, I just like it to have a picture in my head of what it looks like.)