Imagine a rider on a horse. Imagine the rider is hit with involuntary movement. Will the rider be unhorsed?
At first, when contemplating this question I inferred that a rider must be grappling the mount. Involuntary movement beyond the rider’s reach would therefore force a dismount. It all seemed consistent. I chose to ignore that a grappled pair has a movement speed of zero.
Then I found the Mounted Combat rules:
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.
That this rule exists tells me you are not technically grappled to a mount as it describes an entirely different mechanic. The rule makes very clear what happens if your mount is affected by involuntary movement. What I can’t find is what happens when the rider is affected by involuntary movement. It's an unfortunate omission.
Take the simple 10-foot Repelling Blast Eldritch Invocation (or any similar 10-foot involuntary movement). That would seem to eject the rider from the mount, though I’m open to counter arguments. There is no option not to move 10 feet by making a mounted Dexterity save with Repelling Blast. There is no rule that would force the mount to move with the rider that I can find.
Now take the Telekinetic feat or combat action Shove. Upon success, this moves a target 5 feet. Short of a wall there is no option for the rider not to be involuntarily moved. If you shoved me 5 feet while riding a horse I would fall off the horse. A rodeo rider might not. If a rider were considered grappled to a mount, then it would not be sufficient to break the grapple, as the mount would remain within the rider’s reach, but I’ve already concluded the grapple rules do not apply. What to do?
Questions:
- Would a 5-foot involuntary movement affecting a rider force a dismount? If not, what happens?
- Is there any 5e mechanic infantry might use to unmount a rider?
My conclusion is that any involuntary movement affecting a rider of a normal one-size-larger mount forces a dismount. Grapple rules do not apply.
That out of the way, would you do anything different if a creature were lashed to a mount? I ask because in the same encounter that I used the Telekinetic feat to dismount a rider we also had an unconscious character lashed to a horse involuntarily knocked into a raging river. Good times.