How does the Sentinel feat interact with the Mounted Combatant feat?

Question

When a character with the Sentinel feat hits with an opportunity attack against an opponent with Mounted Combatant, does the mounted opponent need to stop moving?

The Sentinel feat (PHB, page 169) gives the following benefit, which is usually clear in its applicability:

When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.

It seems a bit ambiguous regarding controlled mounts, due to the way movement rules work, and the way Mounted Combatant redirects attacks.

The relevant benefit of the Mounted Combatant feat (PHB, page 168):

You can force an attack targeted at your mount to target you instead.

The rules on controlling a mount (PHB, page 198) state:

The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge.

Suppose the scenario is as follows. Person A has the Sentinel feat; Person B has the Mounted Combatant feat, and is on a mount. On B's initiative, the mount goes, and B gets within reach of A and gets hit with Sentinel's opportunity attack as they try to leave. However, B redirects the attack from their mount to themselves.

While the attack of the character with Sentinel was aimed at the horse, it ends up hitting the rider. However, the rider isn't actually using their own movement here; the horse is. Does the horse/rider pair stop regardless, as they are moving together? Is there some weird interaction where the rider is stopped, the horse is not, and they perhaps have to make some sort of save to stay in the saddle? Or does Mounted Combatant negate this portion of the Sentinel feat, rendering it ineffective?

Answer 1

The creature that is hit has its speed reduced to 0

If a creature with Mounted Combatant uses that to make themselves the target then their speed becomes 0 and the mount is unaffected. A rider is not using its movement, the mount is, so it is free to keep moving and carrying its rider with it. The rider has its speed set to 0 so it cannot move, so it cannot dismount, or rise from prone etc.

Answer 2

The rider's speed is reduced to 0, which may or may not do much

As shown in this Q/A, it is disagreed upon whether or not a mount and its rider have separate turns. This doesn't make a significant difference either way but there is one interesting result if you assume they share the same turn (discussed later).

Quoting the Sentinel feat (PHB, p. 169; emphasis mine):

When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.

Here "the creature's" refers to whichever creature you happen to actually hit with your opportunity attack.

The Mounted Combatant feat (PHB, p. 168) states:

You can force an attack targeted at your mount to target you instead.

This means that the opportunity attack will now be targeting the rider, and thus can hit only the rider. If it does hit then the rider's speed will be reduced to 0; however, it will only be at 0 "for the rest of the turn".

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If you adopt the idea that the rider and the mount have different turns, then this will almost never have an effect on the rider. Note that there are a few exceptions, such as the Dodge action, which states:

You lose this benefit if you are incapacitated [...] or if your speed drops to 0.

You would lose the benefits of taking the Dodge action in this case, even though it is not your turn.

If you, instead, adopt the idea that the rider and their mount share the same turn, then the rider's speed would be reduced to 0 for the rest of that turn and thus the rider could not dismount or move voluntarily at all.

The rider's speed is reduced to 0, but depending on how you rule on turns in mounted combat, this can have a different number of effects on the rider.